Without seeing the original images I can't say what the issue is. However, with my concave diffusion principles the image often comes out very low contrast out of camera, and can initially seem soft. I created a diffuser to produce this very low contrast lighting so I could use strong sharpening and microcontrast boosts in post-processing without it looking over-sharpened. This was to counteract the diffraction softening of using quite a small effective aperture. At 1:1 an nominal aperture of f11 i.e. the aperture shown on Canon DSLRs is actually an effective aperture of around f22, and well into the zone where diffraction softens the image.IMAGE LINK: https://flic.kr/p/7VBVgg Sharpening comparison
Undiffused or less effectively diffused macro flash creates a type of natural sharpening. What I mean by this is that you get little bright bits with blown highlights, and deep black shadows. This gives high acutance i.e. apparent sharpness. In luminance values the white is close to 255 and the black close to 0. The problem with this if it's in your RAW file it's baked in. What's more if you try to apply more sharpening or microcontrast to counteract diffraction softening it will look horribly over-sharpened. This is the reason for using low contrast lighting. With what videographers call grading you can add contrast and processing to a low contrast image. Unfortunately you cannot undo high contrast in an image when the contrast is baked into your original exposure. It's the same for overall image and microcontrast. It is far easier to work on a low contrast image in post-processing, and then to finally adjust the contrast to taste.
What I'm saying is that with no flash diffusion for macro flash, or less effective diffusion that it looks sharper out of camera, but you can't do much with the image in post processing without it looking over-sharpened. Whilst these images look sharp at web size if you look at them at 100% view, they are lacking in image detail. By using low contrast but oblique lighting, and then additional sharpening and microcontrast boost in post processing, there is far more apparent detail at 100% view. Before applying this extra sharpening and contrast boost I use noise reduction software (Topaz DeNoise) to remove any image noise (this will act like a key for the sharpening and will create ugly sharpening artifacts.
There's a difference between low contrast oblique light and flat lighting, even if they look superficially similar before post-processing.
Flat lighting is lighting from the front. Here there are no shadows at all, and if there are small bumps i.e. texture, there is no highlight on one side, and a shadow below as you get in oblique lighting which brings out the texture in a photograph. Even increasing sharpening, microcontrast, and overall contrast will not re-create texture with flat frontal lighting.
However, with low contrast oblique lighting from say above, there are still micro shadows below texture and brighter highlights above, it's just that they're not visible yet in an unprocessed image. When you increase sharpening and micro contrast, with say wide pixel radius sharpening, along overall contrast boosts, this texture will become far more apparent.
It's easiest to illustrate it with this image, which I used to counteract mistaken criticisms that the lighting from my diffusion was flat. These are the same exposure of the same Ladybird taken with my concave diffusion. The image on the left has had a moderate amount of sharpening and boosting of the micro contrast, and overall contrast. The image on the right has had more sharpening, microcontrast boost, and overall contrast adjustments. Not how much more image detail is apparent, and notice how it has brought out far more of the texture of the subject, which would not have been possible if this was truly flat lighting.
by Stephen Barlow
, on Flickr
What I cannot say without looking from a full resolution image or a crop from the out of camera exposure is if the images are soft i.e. the detail is blurred, low resolution, whether the image has flat shadowless lighting, or if the light is just low contrast (where better processing would bring out more detail). As I think I said earlier I have tried bond paper as a diffuser. However, I found it absorbed far too much light, so it is possible your flash is firing at nearly full power and not freezing motion. There is a way to test this. Whilst the 270EX does not have manual power ratios on the flash, with many Canon DSLRs you can go into the menu and find the external flash control panel. This allows you to adjust the power ratio in manual, so you can establish what power level your flash if firing at for a correct exposure i.e. if it is firing near full power.
If the diffuser is simply absorbing too much light then simply use velum paper or preferably diffuser gel in it's place. Both are easy to cut, and all you have to do is to use your bond paper diffuser as a template, draw around it with a pencil and cut a diffuser out of one of these other materials. Both velum paper and diffuser gel transmit far more light than bond paper, but they still have excellent diffusion characteristics even with a single thickness. Having said this, if your flash is firing straight through the diffuser at the subject, you might get a bright spot. So possibly you could just a double thickness with a gap, or just a double thickness where the centre of flash strikes the diffuser.