Maybe you've tried this before, but it helped me. I'm not used to these parts failing so often; it's usually the guides that break.
The electric motor repeatedly stops at the same point because it has "learned" to do so. This results in a "dead point." This is somehow related to the carbon brushes.
Mine was "broken" at one point, so I took it out. Then I tapped it lightly while starting the motor (separately from the guides). Eventually, it started working again. I then ran it for a very long time in both directions, reinstalled it, and it's still working.
The tip, by the way, came from my friendly

.