Tightening the nut clamps down the wiper fingers a little tighter which adds more friction as they wipe over the contacts. If it's not working with the nut tightened there's either too much friction in the stepper, or not enough torque in the spring wound around the shaft.

When you had the wipers off, did you clean the wipers and the contact board? And did you clean both the shaft and the bearing it runs through? If everything is clean then you may need more tension on the spring. Other steppers commonly need about 4 turns on the shaft spring to work reliably. Try adding another turn or two to your spring to see if that helps. Too much tension though and the stepper won't be able to step forward.

This game uses magnetic reed switches below the targets:
https://ipdb.org/showpic.pl?id=3213&picno=64914&zoom=1
Try moving the targets manually and see if you can see or hear the magnet reed switch activating. If you put your meter across each switch you should see its resistance drop to maybe an ohm or so when the switch closes. Do they all do that?

/Mark

Might be some useful info here too: https://pinside.com/pinball/forum/topic/anyone-have-expertise-in-reading-midway-schematics

Looking at the schematic piece drsfmd posted in the other thread, it looks like those magnetic switches are normally closed, not normally open. Either way, when the targets swing you should see the switches open and close either visually or with a meter.

BTW, drsfmd if you're able, please consider scanning your schematic and sending it to ipdb.org since they don't have one.