You'll want to check the circuit that sends power to the ball count reset solenoid. This might be an issue with a mis-adjusted score motor switch. Check the following switches:

Score motor switch 2A - green/black wire and orange/red wire
Game relay switch - black/white wire and orange/red wire
Extra ball relay switch - yellow/brown wire and green/black wire
Ball index relay switch - grey/green wire and yellow/brown wire
Outhole relay switch - yellow wire and grey/green wire

Make sure these switches are clean and adjusted properly...

Yeah, the current copy of the schematic on IPDB is definitely an eye strainer...