Remove the stop screw and rack the bellows all the way out. The rails on the bed will simply disengage from the pinion gears. You may not need to do more than re-engage the rails, making sure that they start evenly. The problem is when one rail is a tooth or two ahead of the other, which skews the bed making it rub.

If there is a place that needs sanded down, as Richard implies, sand until you can rack the bellows smoothly and then use a little tung oil or the like to finish the sanded areas.

Best,

Doremus