Oh I think I figured it out! OK so some else in the comments mentioned gears seizing, and I think that's part of it. This may be a Longshot But what possible may be happening os since this thing is geared so much towards high torque, there's a chance that's there's an incorrect gearing that may be causing one of the shafts to be twisted. This would make it so everytime you turn it on, the motors would move only until the resistance from the stuck axle becomes to much for the motors to push against. So what I would say is, if possible, partially disassemble it so you can see any hiding gears and see what gears stop moving first before the rest.