I had a 1953 United Chief Shuffle Alley that only played 6 player games when I first purchased it. I cleaned all the score reels and stepper units. It continued and would only play 6 player games. I visually inspected everything I could. Lo & behold, one of the wipers on the back side of the score motor was broken with the contact being missing. I soldered on a new piece from some parts I have laying about..... so that the wiper would actually make contact, and it worked just fine.
That machine is no longer in my rec room....as someone wanted it more than I did.
A couple of months ago, I bought a 1954 United 11th Frame Shuffle Alley. I didn't know if the machine was operable or not. I again cleaned all the score reels and stepper units. I had to repair a couple of pins, do some cleanup and look over the contacts. The backglass had some paint flaking as well. I bought some of those $.50 paints from wally world and did my best. I sure wouldn't even think of doing this type of paint touchup on anything but an inexpensive machine. I had only paid $250 for the shuffle alley. The machine played just fine and I have a very low cost playing shuffle alley.
I also have a 1957 United Bowling Alley Ball Bowler. It was inoperable when I purchased it. All the score reels and stepper units were cleaned. I had to replace a nylon gear in the score motor. It still wasn't operable.
The culprit. One of the switches on the score motor was out of adjustment.
Its now operates about 98%. I still have a "flyaway" bowling pin and some occasional 10th frame scoring issues.
So I am far from an expert on bowlers, having some successes, but still get stumped on some issues.
All I can say is to visually inspect everything with a clear eye on everything. manually try all stepper units to see if they are operating as they should.