The first thing I'd check is your fluid level in the hydro unit. The hydraulics run off that (the hydrostatic pump), and if the fluid is low, you won't have much lift. Check your hydrauliccylindar for leaks. If fluid is bypassing the seals, you won't have much lift. Lastly, check the control valve and all lines for leaks. It should raise you (provided you don't go beyond the factory rating of 500#) without a problem.
Good luck - Rich