IF your knob will not turn 360 degrees all the way around - it means you can NOT utilize the 'lock' position to hold up the deck at its highest position.. That is ASSUMING that the cam (Part #5) under the tractor is still spinning when the knob is turned from up top.
Tractor off, mmm off, 3pt UP - climb under it and look at the cam set up.. you will see the 'ear' (the metal tab between parts #11 and #12) that hits the cam - when it doesn't turn freely this ear is actually hitting the cam at the 1st (highest mowing) setting.
By raising the MMM up a tad more (mine needed approx .25") you can utilize the lock position as it was designed (at the widest part of the cam).. I am not sure how you adjust a hydraulic lift
I have attached a pic of the lift mechanism for the mechanical lift - I am assuming the knob set up is the same though