Quick update - I've had all the motors off and replaced them. The motor is working now but intermittently. I have just read the support article about bending up the wires so I will try that. Will keep you posted.
Thanks @robolink_wes - I've done this and no improvement. I also swapped the motor to a new arm and the same motor didn't work on that arm, so I think it is probably defective. How do I obtain replacement?