Two thoughts come to mind.
One, balls magnetized and not rolling into position right.
Two, plunger that kicks the ball up and out of the ball trough, is a very wee tiny bit too high and occasionally traps the balls. The whole coil/plunger assembly can be raised or lowered by loosening the four screws on the end of the ball trough. Don't go too high or too low.
LTG : )