I just cleaned, re-greased mine in my 91. There are 4 issues. First, the tracks are not aligned properly. Second, Low battery voltage. Third, old (waxy) grease in the channels. Forth, old grease in motor/cable assembly.
Mine turned out to be combinations off all of them. Used brake clean and an old tooth brush to clean all the old great out. Re-greased with a thin coat of marine bearing grease. Took some time to re-align the tracks and motor assembly. They zip up and down nicely now.
I have seen in some cases where the motor assembly needs to be rebuilt. Mine were not to bad to start with...
Jeff