1984 Chrysler 5th Avenue Window motor

My 1984 Chrysler 5th Ave driver side window stopped working. The motor works but the window will not roll up at all. Nothing but the sound of the motor. I checked inside and replaced the motor with an exact motor from a 5th ave I obtained from a junk yard, but still no results. The teeth on the motor and the other part inside the door where the teeth connect with the motor are okay and are not stripped. I tested the motor before installing and the gears are working on the motor. Are the motors just too old and weak to push up a heavy window? In the past the window would roll up only halfway, but that was due to the rod inside the door where the window glides needed lubricating and worked fine after that. But not this time. I am puzzled.