The problem could be in either the score reel, or in the circuit that's driving it. To help isolate the problem you could jumper the coil on the misbehaving score reel with the coil on another score reel that is working properly so that both will get the same pulses. One coil terminal is probably common between all the score reel coils. Just connect the non-common terminal of the misbehaving coil to the non-common terminal of a good one. Maybe pick a score reel that's not next to the misbehaving one to avoid complications when carrying over from 9 to 0. Then make the game do whatever it needs to do to demonstrate the problem. If both score reels demonstrate the problem, the problem is upstream in the driving circuit. If one score reel works properly and the other doesn't, the problem is in the score reel.