Hi
Get yourself some fairy liquid and water mix.
Spray a little on the fill nipple area.
If bubbles are coming from around the threads tighten the threads and test again. If it still leaks empty the bottle, remove the fill nipple and put new PTFE tape or locktite on and retighten, test again.
If the bubbles come out the valve input then you have a poor seal. this could either be a bid a dirt is keeping it open a smidge so it leaks, often solved by putting the bottle back on the fill rig and filling a bit more.
If that does not solve it it will need a quick rebuild to replace the sealing o-ring that has perished / got nicked.
All of the above can be done by yourself if you are happy and conifent to do it !
If not send it to your nearest store or visit a local field and let them do it for you.
The golden rules of air sysytems are :
Never unscrew anything when there is pressure in the bottle.
Never put oil into the fill valve.
Regards
Ed