This might be a mechanical issue with the player unit. Check if you can manually advance the player unit by pushing the solenoid plunger. It should smoothly advance. If it doesn't, then the player unit will need to be cleaned. Most likely it's old grease that is preventing the unit from advancing properly.

Other issues could be a faulty switch in the player unit solenoid circuit. Do you have a schematic for the machine?

Find the add player unit solenoid on the schematic and work your way through the switches. This circuit includes a lot of switches, so you can always use a jumper wire to bypass suspect switches... Also, check all the switch stack screws on the player unit to make sure they're tight...