Small Engine Problems? Ask an Engine Mechanic for Answers ASAP
OK I see you have changed fluid and filter which would have been my first question :)
The only other things that may cause that issue would be the hydraulic pump itself - likely the pressure relief may be bypassing fluid, a worn hydraulic pump, slipping coupler between engine and pump
there is no rebuild kit for this or the hydraulic valve - only thing to do if cleaning the pump does not resolve it would be replacing the pump.
other than slipping lovejoy coupler or the pressure relief bypass in the pump or the hydraulic valve leaking - restricted flow from the wrong filter, there's just the pump left