It sounds like your ball count unit is not resetting to 1, it is staying at game over. There should be 2 solenoids, one that steps up and one that resets to 1. Try manually actuating the plunger of each to see if you can get the unit to reset. If so, then try to play a game. If it plays one game but won't reset after, then you have to look at the solenoid that resets the ball count unit.