as you know, the pto must be operating for the hyds to operate.
if the oil is milky and dirty, the control valve can stick.
if you remove the round cover where the dipstick is, there is a long rod from t he top cover to the pump. as you move the 3pt lever, this rod moves the control valve ina d out of hte pump. if the oil is gritty and dirty, the valve may not move fully in or out.
post back some more info