Maybe dress your armature plate and brushes tips. (I dont have that one) One aide of thw gaps can go low over time. Brushes can develop an angled tip, turn and hang in a gap too. Or both.
Or a thrust washer? When you reverse, it pushes the shaft the other way (up or down) as well. It can cause heavy drag and a washer helps. This can be a tiny roller bearing vs washer on some motors... bad bearing? hard bearing grease etc.. Ive added thrust washers to motors that didn't have them. It's a matter of how much thrust play is there id you can fit one.
Machine shim washers are hard, thin,and milled flat to spot on thickness vs any old washer. Brass is usually pretty good too.
Armature shaft bushings oblonged, the shaft can lean enough the armature winding body can scrape the field plates. (can be directional)
I guess axle bushings could give bind on a worm, I just haven't seen it . Normal gearing it happens. You have to watch gear posts and gear post holes, watching for leaning off plane from its mate(s) etc.
Quartering off a hair or rod issues can also be directional where you might not expect it. If taking rods off stops it, that's it.
