I have a quad. Motor 4 doesn’t want to spin. However if I switch motor 4 and it’s esc with motor 2 and its esc where the ESC connects to the fc, both motor 2 and 4 spin. I have interchanged the motors, esc’s and both the motor and esc check out ok. but when the fc is connected to the esc for motor 4, motor 4 won’t spin. It just jiggles a bit like it is trying but can’t.
I solved the problem, I did not have a ground wire to each esc. When I added a ground to motor 4, it now works.