Check that a ball is not getting "stuck" in the trough. As the trough wears, it forms divots where the ball resides.

Next time the problem occurs, turn the game off and open the front door. See if the ball that is supposed to be in the ball shooter lane is stuck in the trough.

Another test is that next time it happens, shake the game side to side and see what happens.

Very common fault - The only way to fix this properly is to dis assemble the trough and use a file to file out the divots.