[beiju]

Are you stopping the motor manually or via sensor? It could be that your reaction time isn't enough, and your window motors are straining to continue after your ball holder hits a mechanical stop. Try adding a microswitch (they're really easy to mount and wire, and I can help if you have any questions) and programming whichever button lowers the system to stop working if the switch is pressed.

And of course, keep changing your power level in decreasing increments until you get the level that works. One year, we had a system that only worked properly with a very specific power level that was carried out to four or five significant figures, but once we found that level it worked beautifully and didn't need changing even at the competition.