What car for a start?

Generally, it'll be down to either the motor getting tired, or the mech being out of line (ie, needing adjustment). In most cases, dousing the front and rear channels in the door frame with plenty of WD40 will usually help a lot. Stripping the door down and coating the runner and ends of the mech arms with LM grease won't hurt either.

You need to see if there's any/much play in the window - grab the window when it;s half way up/down and see if there's much in the way of front to back movement in it that might suggest the mech needs adjustment. Compare it to the opposite glass to see what difference there is