First off, do you have dry skin and are you in a cold climate ? As the weather changes, if you have dry skin, you thumb will be much smoother (drier) and come out of the ball much quicker. Once it gets warmer, you thumb will get a bit moister and the thumb will drag a bit.
Also, many bowlers are now using more forward pitch in their thumb. You can try a bit more forward pitch in an older ball and see what happens. Basically, you can go more forward until you hang up, then back it off a bit. Be sure you are not grabbing the ball, otherwise with more forward pitch, you will not come out of it.
Edited by Stan on 3/14/2011 at 3:37 PM