The first thing to check is does the unit subtract credits if you manually operate the plunger?
If it is mechanically OK then here's the schematic snippet below. Check the ZERO POS. "REPLAY" UNIT switch is closed when there are credits > 0.
You say the rest of the game works OK so you can eliminate the Motor 2C and S contacts as they also also operate the ball count and AX relays (which we can assume work).