just recently bought an ultra, guy filled it up for me no bother when i picked it up.

i got myself a 300bar bottle today and ive attached whip to gauge and my fill adapter to the whip. When i open the valve on the bottle with the air flys out as intended, i then cock the gun put the adapter into the gun and try to fill it up but my gun pressures not going up. I know i have no leaks and that the bleed valve is closed....anything im doing wrong here? its my first pcp>??

THANKS