Just fixed that exact issue on my Jumping Jack an hour ago. It also worked during reset but not between balls/players. And *sometimes* it would work if I advanced it past player 2, but not always. I jumpered everything in its path (two switches at the player unit,3F and 4E, tested fine as did the jones plug connection and wiring all the way to the score motor.) I found the issue at the Score Motor, switch 1A middle switch out of 3 (advance player unit coil.) I believe it was a Brown/Yellow/Red wire and a Slate/Green wire, though I could be wrong. It buzzed fine with my meter but did not work until I jumpered it. A good cleaning and adjustment fixed it. JP may be different because it uses AX/BX and JJ uses a control bank.

I also suspected the first ball relay (house ball relay) but that worked as it should, activating when the first point on the first ball was scored.

I don't have JP schematics at home with me but I have them in my studio, I'll be there tomorrow and can check them out if no one else does beforehand.

That's why I also struggled with mine. The game is new to me so I had no history with it. (beyond it belonging to a radio engineer that sprayed contact cleaner on everything, greased the coils and adjusted almost all the switches. Great.) It advanced perfectly during startup. I focused on the player unit thinking SM was too early in the chain. So I wasted way too much time on that player unit. Had I just pulled out my jumper wires earlier and tested all the way to the score motor, I would have had an early dinner.

That said, it could still be the player unit on yours

PM me tomorrow if you still need the schematic info. Unfortunately, I recently put the game away so I can't crack it open to play along with you.