Despite checking all the cheap options like drive chord slipped down the back of a pulley or motor/gear lube, this is usually a problem with the top or more likely the bottom sliding rail. Needs lubrication or replacement.
General crap collects in the bottom runner and wears it out.
Careful application of silicone spray may fix the problem on a temporary basis.
Replacement is quite expensive as the kit includes top and bottom rails and a new drive motor! I have a collection of new motors which I have never needed to fit!