As Gav suggests, the ball count unit is your first item to check.
The more complicated item will be the player unit. This is the large unit in the headbox. There are a lot of contacts, cogs and armatures. This unit is responsible for moving features, lamps and scoring from one player to the next. If the switch stacks become loose, you can have all sorts of issues with this unit. Tighten each switch stack - there's a few to do here, and you will need to unclip the unit to access it correctly. I feel the problem will be in this area.
The easiest way to troubleshoot is to test the game with 1 player only. Problem persists, it is more likely the ball count unit.
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