To start at the beginning.
Close the bleed valve on the line from the tank. This could be a 'turn' screw or an automatic push button. If you cock the rifle then connect rifle to tank and slowly open the tank valve to let air into the rifle, the guage on the tank will show pressure increasing as the airline pressure rises. There should then be a 'click' as the rifle inlet valve opens and air goes into the rifle. But if rifle is empty the click may not be there and air will flow into rifle straight away. The gauge on the rifle, assuming there is one, should now show the pressure of the air in the rifle and the tank guage the same pressure, almost. I think the PH6 charges to c 160bar? Anyway, don't go above it or even go to about 120 bar at this point. If during this operation you can hear air escaping then you have a problem to investigate. If you manage to get air into the rifle and then close the valve on the tank (at about 120 bar on the tank guage) and open the bleed valve on the line between rifle and tank then, all being well the air in the rifle should stay there while the excess air in the line between rifle and tank should escape. If all air in the rifle escapes at this point then you have a problem with the inlet valve on the rifle as it is not sealing to keep the air in. Air could escape from the inlet valve mounting (usually a dowty washer) or from the internal seal in the valve
If air is going into the rifle but not registering on the gauges you may be able to tell where it is going: likely escape routes are from where the gauge screws into the rifle, if it has one, or down the barrel if the rifle exhaust valve is faulty or even a fault in the air line between rifle and tank.

Good luck
Cheers, Phil