Ey up
Morning all!
If the line "POPS" into place ,then you continue to fill,, the bottle gauge goes up,and the gun gauge doesn`t,,,it sounds like the fill valve is not letting the air into the gun.
All you will be charging is the line. You should be venting this off to allow you to disconnect ?yes ?
if it wont fill the gun, get it looked at by someone in the know, Gunshop/gunsmith etc
Just be safe and remember that the pressures involved are huge!. You shouldn`t be afraid to fill the gun ,or at least try and get it to fill, but If you are really uncomfortable with it,,no am