it is a simple system with the front section of a two section gear pump suppling the implement circuit. it is possible that the pump has a problem or that the pump inlet oil is restricted in some way
are there any noises like marbles in the pump or does the oil appear aerated or milky. here is the pump performance guide to check the pump
you could also have low pilot pressure going to the main control valve to operate the lift spool. to check that, you would either have to remove the batteries to gain access to the test port on the transmission pump which supplies pilot oil or put a tee into the pilot line that controls the lift function. should be about 430 psi at idle and about 460 at high idle
another cause could be leaking seals in the lift cylinders you could isolate each cylinder in turn and see if that makes a difference