A worthwhile thing to check is to disconnect the motor, and hook up a meter to the motor leads. If you have a short burst of power to the motor when you open the screen door, and then the power cuts off, you are going out on motor overload. The symptoms are exactly the same, it does not move, but it takes you to different things to check.
Mine did that, and I ended up loosening the nuts to hold the unit onto the chassis, and then it ran perfectly without going into overload. I put some shims under the nuts and that solved it.