I don't have any experience with that assembly, but looking at the diagram in the manual it appears there's a set screw on the motor shaft. Well, it's not actually shown on the illustration, but it is mentioned in the parts breakdown. Here's a screen cap of the pages i was looking at, note that the part list is referring only to the motor assembly on the right. Screenshot_20200820_124249_com.google.android.apps.docs.jpgFurther up the manual shows the breakdown for the moving gate assembly. Anyway, number 5 is on the parts list for the motor assembly is a set screw--I'd try to locate that set screw and confirm it's tight, or tighten it if it's loose. Again, it's not shown on the motor assembly illustration, looks like it's probably hidden at that angle. Anyway, if the set screw is tight, I'd also inspect the linkage between the motor and gate for anything broken or loose.

Could be an internally stripped motor, but I'd try to verify everything else before replacing it. If you need a new motor, Marco has them in stock here: https://www.marcospecialties.com/pinball-parts/14-7941-3

Good luck!