"usually" the rollers get worn which then lets the door drag instead of roll. Take the door off and check out the rollers, if they aren't worn flat then lube with any household lubricant (wd-40 etc) and put it back in its track. You can also adjust the rollers, there should be a screw in the end of the door frame, turning clockwise will cause the roller to move down. This will allow more "gap" at the bottom (if the door is dragging). You can also replace just the roller assy.