I am trying to run this through without a schematic. It sounds like you need to look at the schematic and trace the circuit from the 10 point relay to the 10 point reel. Did you try the second player test? When you trace the circuit path from the relay to the reel. See if there are any contacts / switches in that path. Then put a small piece of cardboard like a cereal box or heavy business card in between the contacts of every switch. See if the locking relay stops. If so, then pull the cardboard one at a time and test each time.
There are many small little things that can cause a problem like this. Hope the cardboard test helps.