If this dipstick shows a good level, next step would be to chech the strainer to see if it is clogged. For this you need to drain the fluid and remove a bolt from the side of the transmission, that bolt goes through the suction line to the hydraulic pump, it is on the right side of the tractor and hidden by the tire, I always pop my tire off before trying to remove it. Also when you engage the lever in either direction do you hear any change in the noise from the engine area?