Is it a powershift or Quad-range or Synchro-range? All of them are adjusted with shims under the pressure regulating valve. The location of the valve is different depending on the transmission. You can only adjust pressure on entire transmission. If transmission pressure is good other than pto, then there is likely a leak in the pto circuit. Put a gauge in the transmission system pressure and see if the pressure drops when you engage pto. It should dip briefly and then recover.