left down in the original motor, then the plastic cable is not threading or wrapping smoothly enough around the gear at the bottom. The motor has an automatic cut-off when a certain amount of tension on the cable is exceeded. That is how it turns on and off normally when the cable is fully retracted or extended, and one of the reasons they wear out so much, because of this design that puts "force" on the cable at its endpoints. ANYWHO, remove the mast and cable while leaving the motor in place and rethread it a couple of times (two-man job) until it goes all the way up and down. Inspect the tip of the plastic cable and make sure it doesn't look snapped off.
