What's the key to rendering eye color?

Hello all,
I have a problem that has been frustrating me for some time now. I take alot of photos of people and the exposure seems good, however, the eyes always seem to be dark with little to no color visible. What am I missing? Do I need to use a polarizing filter? Or, maybe I need to shoot light straight in? I don't know, this seems like a pretty basic concept, so it's making me pretty frustrated that I'm having so much trouble with it. I greatly appreciate any and all advice you can give. Thanks in advance.

Without a sample shot to see, I'll just take a stab at it...

What is likely happening, is that the persons eyes are in shadow. Perhaps a fill flash might bring out the eye colour more. Or you could use a reflector to bounce the sunlight back up into the subjects face. Be careful here or your subject may look strange (no shadows on a sunny day!)

A polarizing filter will work great if you are fighting glare instead of darkness.