Check for mud, dirt, etc. encrusting the linkages. Clean, then spray area with WD40 or similar. If this doesn't work, adjust large gold or silver hexhead bolt next to left rear tire. First loosen the locknut, then tightrn the bolt little by little until you can turn the knob to Top.
Adjustment on the rear of the tractor between the drivers side rear tire and case. Its a gold anodized bolt. Undo the jam nut turn it one full turn and tighten the jam nut. Your hight adjustment will go to TOP.