Hi Ross,
I think it can be caused by a couple of things:
1. Hair/dirt being tangled up around the motor axle.
2. The propeller being pressed tight on the motor after a crash
3. The motor bearing being depressed into the motor housing.
Points 1 and 2 are pretty easy to fix and I think you already tested that so we are left with point 3.
Depressed bearing is something that can happen when the crazyflie crashes upside down. You can identify it by looking at the little cylindrical part sticking up from the top of the motor. If it lookes depressed compared to the other motors, that is the issue.
Most often this can fixed by pulling it out with a wirecutter/nippers and a a pliers.
We made a (as usual shitty
) video of how this can be fixed
http://www.youtube.com/watch?v=cgD76HyMLrs
This can be prevented by putting/gluing a spacer between the motor and the the propeller, so the propeller can't depress the bearing, but we havn't had time to look into it. Something I think we should make a higher priority now. If you have any ideas of any good spacer please share.
