My first inclination is that you are squeezing the ball. That is the quickest way to kill rotation. It causes a delayed thumb exit, which cuts the time between thumb exit and finger exit. This shortened time frame is too short for the fingers to have much effect on the ball.
If you have a professional coach associated with your bowling center, or available in your area, have him (or her), come down and drill you on the basics of free arm swing and proper release.
They may also find other issues for you to work on to improve your overall game.