Hi Peter, my 2 penny worth.
I think you have two problems;
1, is that you have a leak depleting air from the chamber, which as mentioned above could be the reg or the valve, or even any of the housing o rings.
2, the reg is not filling when the pressure in the chamber reduces ( leaking occurs)
The reg contains a small aluminuim piston with a delrin insert, this insert acts as a valve seat against the supply of air from your cylinder and as such operates against pressures upto 200 bar. After time this delrin insert wears and can form a little nipple (mine did)
As your chamber pressure is slowly depleted through the leak, and the piston moves away from the supply jet, the little nipple is still making a seal and not allowing the cylinder air in.
IMO the leak is also coming from one of the two O rings on the piston inside the reg.
Chris will be able to supply you with a reg piston and O rings, but you will need a reg checker and a special forked screwdriver tool to set it back to your prefered pressure (prob around £30 for the piston & O rings)
A set of O rings for the housings is about a tenner, and I would fit in a new firing valve whilst its apart, they're not expensive.
Good luck with your repair, and let us know how you get on.