I had this with my passenger a while ago. Voltage was getting to the windows and I could hear it click but no movement. I took the door card off and tested at the motor for current. I then took out the window and then ran the mechanism back and forwards a few times. A bit slow at first and then it sped up. I then rubbed some candle wax down the inside of the runners. Put the window back in and it's spot on now - nice and fast and works a treat. I do think it happened due to underuse so I now try to use both windows at least once every few days when I'm driving him.