I believe there is some confusion here on what type of mower lift we are talking about. The standard lift system uses the three point hitch linkage to raise and lower the mower. You need to use the rockshaft control lever to operate it. The optional lift system uses a separate cylinder that needs to plug a hose into one of the hydraulic couplers on the right side. You use the loader joystick to raise and lower the deck with this setup. I am guessing this is the type gtrman has. I don't see how the height adjustment knob can prevent the deck from going up. It can prevent going down or prevent holding it up in the highest position if not adjusted properly. the adjustable turnbuckles for either setup should be adjusted as short as possible without the arms hitting the stops when all the way up. If you do have the optional lift system, the three point hitch has nothing to do with the mower. If you have the optional one, I would try to switch the hose that plugs in the couplers to a different plug and see what happens. You will just need to move the lever in a different direction to operate it. Good Luck!