Cooling air can give quite a large drop in pressure as it returns to room temperature . If I fill my rapid bottle quickly to 220bar after sitting for 10 mins the pressure can drop down to less than 200 bar .

Make sure you have a true fill by pumping to 220 bar then let the gun sit a while for the air to cool than top back up . This may take more than one top up to get a full 220 bar fill .