Are you new to the rotary cutter or quilting?
I know the rotary cutter really takes a lot of practice in the beginning. I've learned I have to really have a firm grip on my ruler or it will shift as I cut. And I have ruined a good chunk of fabric, one reason I got into making crumb blocks-to use up all my mistakes.
If you don't have a quilter or LQS handy to assist maybe some books from the libray? If you live close enough to a library-I know some people live way out in the boonies and don't want to mess with driving to the library. There are some good tutorials on youtube this is Pat Sloan's
http://www.youtube.com/user/Harleykn.../6/LjQNY9j_PIQ
If it's a physical problem maybe use a cutting system. Someone has mentioned the accuquilt but there is another one that might be good:
http://www.quiltcut.com/