Most failures to lift are the result of a badly leaking hydraulic system, or a stuck control valve.
With tan implement on the hitch,remove the side plate and with the engine running and the PTO engaged(UNDER NO CIRCUMSTANCES PUT YOUR HANDS INTO THE HYRAULIC COMPARTMENT WITH THE ENGINE RUNNING1), pull the lift lever completly up and look up toward the seat to observe the lidf cylinder. There should be no oil coming from it other than an occasional drip. Using a mirror and flashlight look into the front right hand corner of the hydraulic compart for a vertical tube the supplies fluid to the liftcover/ lift cylinder. Is the fluid streaming from the tube? If there is it"s leaking. next move the lever down and then up and observe the "H" shaped control arm move back and forth to actualted the control valve submerged in the fluid. the lower part should move smoothly forward to lift and smoothly backward to drop. Listen for noise and a lot of turbulence at the aft side of the pump. This could indicate a spartialy stuck open releif valve. Similar observation around the pump side plates are indicative of a blown pump side plate gasket.
Finally,with the engine shut down, reach under the oil surface at the lower part of the "H" shaped control for and feel around fro the "T" shaped control valve stem. mae sure it is attached to the control fork and then slide it orward and pull it aft. It should move about one inch for full travel from full back to full front and should feel smooth.
Do these checks and you be able to determine what the problem is.