[MrForbes]

We had an interesting issue with one of our motors like that, the terminals were somehow in the wrong position. Loosing the clamp screws all the way, and pushing them in, seemed to fix it.

But yeah, make sure the wires are stripped to the correct lenght, and fully inserted, and the screws are tight. Also it might help to zip tie the wires to the motor, or at least to something that is attached to the motor, so that you are not relying on the terminals to take the load of the wires flexing when a mechanism moves.