The heght adjustment "Top" position holds the deck in the fully raised position regardless of the 3 point hitch position. Raise the deck completely and turn the height adjustment to "Top". If the knob won't turn to "Top", do the following:
Looking at the rear of the tractor, just inside the left rear tire is a linkage that controls the lift. There will be a gold colored bolt that faces rearward. Loosen the lock nut on the bolt and screw the bolt in one turn. Raise the mmm all the way and see if the knob will turn to the "Top" position. If not, turn the bolt another turn and try again. It shouldn't take more than a few turns of the bolt unless something else is out of adjustment.