It's hard to say why you are not getting replies, without knowing what you are sending out. The first thing that matters is if you are offering to pay or looking for TF shoots. I suspect it may be the latter.
If you are looking for TF shoots, then you are offering to pay the model with images that they can use on their portfolio, images that will show them in a good way and will help them obtain other shoots. How good, and how relevant, is your portfolio? If a model you message looks at your portfolio, will they see images that are at least as good as those already in their portfolio? They aren't going to be interested in working for anything less as they won't want to reduce the quality of their own portfolio. Also, have you got model images on your portfolio? Some people start with general photography, and while they may show the skill in taking landscapes or whatever, they also tell the model that you probably have little skill in directing a model shoot, using studio lighting etc. At least have images of people in your portfolio, even if they are family members and friends, but don't use snapshots, they should be carefully shot, controlling lighting and pose etc. Whilst a character portrait of your grandfather may not be completely representative of what you are looking to shoot with the model you are contacting, if it is well lit, well posed and well edited, it shows directing and lighting skills that are relevant.
Your portfolio is your shop window and you need to display quality products that a model wants, if you want them to work TF. If your portfolio is not yet up to standard then you will need to pay models for a while, they then get something they can use in return for their efforts (cash). Working with more experienced models is recommended if you have little experience directing, as they know their job and you just need to tell them the look you are after and they can rattle off a variety of good poses without further direction. A newer model will need help with their poses, so a combination of inexperienced model and inexperienced photographer will not produce great results.
I apologise if you are experienced with working with models, and have a good portfolio already, I am basing the above advice on what I normally find when somebody posts your question.
Assuming that your portfolio isn't lacking, there are some other factors that can come into play. As mentioned above, check that you are contacting models who regularly log in and so will actually see your message sometime soon. A lot of models also rely on references, and the catch 22 in the system is that it can be hard to find models when you have few, or no, references but you don't get references until you do the first few shoots. Again, the solution can be to offer payment as cash is a big incentive to models low on funds. You can also allow them to bring a friend or relative as a chaperone, if it makes them feel more comfortable (although chaperones can be a thorny issue and can cause problems on a shoot sometimes - the flipside is that a good chaperone can be a big help with holding reflectors etc.). Another good way around the lack of references is to hire a studio, where the model knows the studio staff will be around the whole time. This means that they have the confidence of a third party present, but the third party will not interfere with the shoot. This is also useful if you are not expert in the various lighting setups and setting up lights, as the studio will have somebody who can assist with your setups.
When first contacting a model keep your message short and professional, outline what you are planning to shoot, where you plan to shoot it, how long you expect the shoot to last and your offer of remuneration (whether in cash, prints or digital images) along with anything else you feel is relevant. Do not use over familiar language, it may seem obvious but some photographers call the model "love" or similar and end the message with a couple of X's. Some models won't care, and some use such language when contacting photographers, but others will see it as unprofessional and a sign of a "GWC" (guy with a camera, more interested in seeing naked flesh than in the actual photographs) and not reply.
As for other avenues of finding models, if you are confident of your abilities in directing then the world is full of potential models. you can even get lucky and discover a natural talent who can give great results without much directing. Where can you find these potential models? Look around you, and you will see them walking down the street, behind the checkout at the store, in your local bar and anywhere else that you go. In this fame hungry world we live in, a high proportion of young women dream of being a model, a pop star, an actress, whilst working behind the counter of McDonald's or wherever. Approached properly, many will jump at the chance of doing some modelling.
The important thing is not to come across as a sleaze, or GWC. Approach your chosen potential model, quickly explain that you are a keen photographer. that you think they have an interesting look (if it is their style of dress that appeals, then mention that) and you would be interested in taking their picture sometime, give some very brief details of what you want to shoot (it helps to be clear that you are not looking to shoot topless or nude) and that you would be happy to provide them with copies of the results. Do NOT try and talk them in to it, or persuade them if they are unsure, just hand them your card (cheaply available from online printers and always worth carrying with you) and suggest that they contact you if they are interested and want further information, then leave unless they are obviously keen to discuss it there and then. Let them think about it and many will get in touch. If you have a good website, then you can put the address on the card so they can see more about you and your work while they are thinking about it.
Your portfolio is important, it is what shows your abilities as a photographer. Be very picky about what you put in it, quality is more important than quantity, just show your best work. Keep it relevant to where you show it, so concentrate on people pics for sites like MM but you can cover all your genres on your website. If a model site has the facility for folders in the portfolio, it doesn't hurt to show all your model pics as individual images with a folder at the bottom for "my other work", where you can showcase your best images from other genres, for those interested.
Try not to miss out on opportunities to grab pictures of people, it is old advice but always carry a camera. You don't need to arrange a full model shoot to get good images of people. If you see a person with a lot of character, don't be afraid to indulge in some street photography and grab some candid shots, they can make good portfolio images (suitable for model sites as well) or even stop them briefly and ask if they would mind posing for a couple of pictures. I have never had a bad reaction to this, the worst is that they just say no and move along, in which case accept that and don't push it. The fact is that many people take this as a big compliment, even if they say no. You will get a good percentage of people who say yes, as they get a good feeling from it, but don't overdo it as they won't usually want to stop for long. Again, business cards come in useful as they can e-mail you to give you their contact details and you can then e-mail them the best result. If you are interested in doing an odd shoot for money, this can pay off as well as they will show the image around and it can throw up enquiries about doing portraits for someone they know.
Shots of kids are always appealing, and again suitable for model sites, but sadly there is a paranoia these days about photographers and kids, fostered by the media, which just didn't exist when I started photography. Back then, taking candid shots of kids in the park was acceptable, these days it is not recommended. However, if you start off shooting your own kids, or those of friends, then you can have shots on hand to show people (using a tablet you carry with you). If you see a photogenic looking kid doing something interesting, then approach the parent for permission to grab a few shots. If you have business cards, a good camera round your neck (an iPhone would send out the wrong signals) and can show a few cutesy images of kids, most parents would feel a rush of pride that you feel there kid is a suitable subject. Again, some will say yes and don't push it if they say no.