Does your Player Up Unit advance when the ball drains in a multi player game?
Fireball Ball Count (resized).jpg
On a one player game pulses come up through the Score Motor 3B switch to advance the Ball Count Unit at the end of each ball. The Coin Unit is in the position shown in the schematic and the Player Up Unit isn't involved. On a multi player game the Coin Unit wiper has advanced to another position depending on how many players have coined up. In that case the pulse has to get through the Player Up Unit and through the Coin Unit (except in a 4 player game).
If your Player Up Unit doesn't advance when the ball drains pulses never get through the Player Unit to the Ball Count Unit.
Williams used essentially the same circuit. More explanation of how it works is in this topic: https://pinside.com/pinball/forum/topic/williams-grand-prix-revival
/Mark