I would put my meter in continuity test mode and clip my leads on the switch blades starting at the end where the wires connect. Try to close the switch by hand. If it won't close, connect the clips to the blades closer to the contacts and test again.
Could be a broken blade inside the stack.

Make sure that the contact isn’t loose it might have to be re-peened (light hammer tap).

Have you filed the contacts? Cleaning may not be enough.

What about if you move score motor yourself and watch... is it really making good contact? You can take a slow motion video with your phone and watch it in action on a reset. I think this is simpler than your making it... if a switch is not working... 99.5% of the time it’s because it’s not making contact. Until you rule that out, that’s the problem.

Switches could probably be cleaned better using a metal polish and a qtip. Like Brasso. I avoid files and low current switches, usually not needed.

Post a snip of the schematic showing the ball count step up coil and associated switches.

Keep in mind the holding a switch closed through and entire motor cycle could involve other switch pulses that are blinded by other score motor switches or by other relays dropping out earlier in the motor cycle.

Yes, turn the game off and advance the score motor by hand. Use the meter test discussed above to verify continuity.

You can also unplug the motor service jack but you don't want to leave the score motor sitting anywhere with power on for any length of time. As you can burn up solenoids.

For contacts to connect - In the order to check:

1. The contact points have to be clean - use a flexstone, emery board or fine sandpaper
2. When activated the long blade has to move the short blade. Looking like the contact points are touching ISN'T ENOUGH.
3. The contact points have to make good connection with the blades. May need to be repeened or even soldered on the back. (As dr_nybble said above)

Have you tried shorting those contacts at the wire soldered ends? if you get identical results as per your shorting the switch blades,and, the machine functions otherwise ok, then your fault must be on the switch contacts,or,there's a break in one of the blades. The switch stack screws are nice and tight, yes?

If the wires from the motor switches and Relay switch to the Ball Count unit solenoid go through a Jones Plug, Bypass using alligator clip jumper leads. If you verify they are the problem; Repair the defective Jones Plug. Clean, retension etc...

DMM isn't enough. You need to know if a switch passes enough current to fire the coil it's connected to. That's why MrBally 's recommendation is better than a DMM.

Lamp test, via Steve Fury :0) (resized).png

Lamp tester (resized).png

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Better still, Steve Fury's method works well:

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That's fine if you want to know if the circuit can pass enough current to light the lightbulbs, but if you want to know whether the circuit can pass enough current for the attached relay or solenoid, use Alligator clip jumper wires.

Nice work! Glad you found it!

Congrats on finding the problem.

Thank you for taking the extra moment to let people know you fixed problem (and what it was). People do not always do that, but I believe it is the thing to do after people help; plus, your solution may help someone else in the future!