There's a nine position switch on the one's reel that closes when the reel is at 9. Check that switch is clean and making good contact. You can identify that switch by advancing the reel by hand with the score reel plunger (game is off) and you'll see it close.
Some of the two player games of that era also have 1st and 2nd Player scoring control relays that might affect your situation so check your schematic to see if those relays are involved but first do the above.