You might be running to the line or your timing is off...those would be the first two places I would check.
I'll add to this.
1. Rushing. essentially, running to the foul line, as that puts your ball behind your body at the point of release, causing you to have to pull the ball as your arm catches up.
2. timing. Lane already covered that.
3. Elbow. Once your elbow comes out to the outside of your body (read: turned away from your body), your shot is going to be tugged. Keeping the elbow locked will prevent that.
4. falling off your shot. At the point of release, you see a lot of bowlers have to pull their extended leg back underneath them to regain their balance (falling off their shot); most of the time, that is a sign rushing, which goes back to timing. The opposite of that would be posting the shot. Good examples of posting the shot would be, say, Brad Snell or Anthony Pepe.
I'm sure others will chime in as well, but these are good for starters.
BL.