You have to loosen the cover mounting nuts to allow the bar to move. If nothing happens, remove the cover entirely and look for a broken adjustment 'tang' that travels along the adjustment screw as it is turned. It is just a short projection on a regular nut that is supposed to poke into one of the large holes at the drive end of the bar. Either buy a new part or if you have access to a welder, repair what exists. Hope this helps!