Video. (of yourself)
It is easy for others to see what you are doing wrong, and tell you.
But it is immeasurably better to see for yourself what you are doing wrong.
Whatever type of roll you are trying to do...
Video yourself and compare it to the commercial roll videos, or compare it to what you see when others do good rolls, or simply compare it to what you think you are doing.
I shoot A LOT of video for my Kayaking Journal website. Last season my roll was getting funky on me after I had got good enough to stop thinking about it consciously. But then when I started missing a few rolls, I couldn't figure out why... until I looked at some video of myself and noticed I wasn't setting up like I thought I was.
Borrow a video camera. Go to a pool session. I think you will be amazed at the progress you can make when you actually know what you are doing and what you need to change.
There are several roll video clips linked from the bottom of this page on the Kayaking Section of Savage Snow. And more are on the way.