Breech seal, piston seal or both.
Probably the easiest springer in the world to service.
I have just done a new to me .22 HC.
It was at 9ft lb.
Took out an extended nose piston seal thingy...no likey.
New , new breech o ring and kept the same spring in it, a bit of bumlube here and there.
11.5 and very consistent.
Just ordered a few more Britseals as spares as I have x4 TX and x2 Prosports.