This is known as an AS relay and is a remarkably versatile and tiny stepper that can be really fiddly to adjust properly. Here are Gottlieb's instructions:

https://www.ipdb.org/files/2233/Gottlieb_1975_Soccer_AS_Relay_Adjustment_A_11492.pdf

You may also find other topics here about this device with a little searching.

I would start by cleaning the wiper board and contacts to see if that helps. Like any other stepper you should be able to pull the armature all the way down, let it go, and have the stepper advance cleanly to the next position. If yours isn't doing that have a close look and see if it's stepping too far (more than a full step) or if it's rebounding and stepping back a bit (or less than a full step). That would help determine what adjustment to make.

Beware that this isn't a device that responds well to a shotgun approach. I wouldn't adjust any more screws until you have a better idea of why it's not precise because the adjustments for back sliding and overstepping are different. Once you know which way to go only loosen screws just enough to get the adjustment you want, make a tiny change and then snug them up again and test to see if things are working better. It may take a few tries to get it dialed in.

/Mark