It will be the pellet probe or the small o ring on the valve stem.
There is an M4 adjuster screw at the back of the bolt with a small locking screw access via bolt gate, for the probe


The get at the valve stem o ring, remove bottle, unscrew Reg, unscrew valve cap, remove return spring and pull out the valve stem with a pair of long nose pliers or a magnet on a stick.

Bb