Someone may have put Locktite on it.
Put a proper size screwdriver in the slot and jar it with a very small tapping hammer. A little WD-40 won't hurt either...
If it is the wheel type adjustment...WD-40 and a good grip it will turn! Work it back and forth ...
If it has the screws to lock it down on each side of the center of the rib...be sure and loosen them first.
