The same thing happened to me and likely you have the cable on side of the sliding trolley unattached. When I opened the door panel and inspected the window regulator, I found end of the cable had come completely off. I probably could have reattached it but when I went to test the motor I also noticed that the friction to actuate the motor in one direction was a lot higher than the other way and I just decided to replace it. I just got the feeling it would happen again and I'd have to open up the door panel again. My thought is something cause the cable to pull out and for $40 i might as well put in a new and not worry about it one since I went through the trouble of taking it apart.