The burst disk has gone slightly or you have dirt in the fill nipple.....
Degas the bottle and take off the fill nipple and clean and lube (NOT WITH WD40) use blue loctite to secure and make airtight the nipple thread. If neeeded get a new set of burst disks.
Dude, seriously, in the nicest possible way...don't post when you don't know what your on about mate.
Burst discs, do not go "slightly"...that's like, partially ignited petrol...won't happen.
Don't advise anyone to LUBE a fill nipple on an air system. You say not to use WD40..don't say why, but don't list every other Mineral based lubricant out there, to not use either. Simple answer is, you DO NOT LUBE FILL NIPPLES.
Your right about the Blue Loctite tho
ICEJOHN
Try and use an adjustable spanner to get the fill nipple out.
Clean the threads in the reg and on the fill nipple, with a tooth brush etc, to get the old sealant off.
Try and get some Blue Loctite (not super Glue) also known as ThreadLock, to put on the nipple before you screw it back in.
Nip it up tight, allow it to set.
If you have PTFE tape, you can use it, but it is not the preferred choice as bits of the tape can enter the air system and your marker...bad.
Wrap the tape the right way (the opposite way, to the direction you sill screw it in) and keep it away from the end of the nipple, that goes inside the bottle, just on the threads.
And NEVER, EVER put any lubricant in or around your air system...just don't...there are circumstances...but unless you know why or what for...do not do it.
For the most part, OIL in the high pressure side of your air system will go bang when you fill it.
You will not find this funny.
People around you, will not find this funny.