Thanks G-benz but no bet as my instincts tell me the same thing as you suggest. Its a neat puzzle as I eliminate what the problem can't be 1. the track is ok and 2. the motor runs fine so that leaves only the manner in which the piece in question moves up and down the slide.

As a last item I am going to rewire the unit outside the door frame so that I can observe the movement under power - other than that I am going to order the part - especially as it is inexpensive.