I haven't tried this myself, just brainstorming, so try it at your own risk.
You could do a little troubleshooting by mounting your TC to a lens, try a less expensive lens first, and then driving both lens and TC with a small screwdriver in the flat screwdrive location, instead of using the camera body to drive the focus train. See if you can detect a difference in the torque between using the TC and lens together and then just driving the lens itself. (You might be able to damage your lens by driving the screwdrive too hard against a stop - so don't attempt this if you can't be careful about it) - i'm sure the motor in the camera body doesn't put out much torque. If the TC requires a lot more force to turn the screw drive, then probably there is something wrong with it.
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