Hi,
Aside from your issue. I would dismantle and reclean the internals and use lubricants more commonly used in air rifles rather than curtain rails. So high moly grease and something like neatsfoot oil on the leather washer. (This is available in a small dispenser from Protek Supplies.)
There is a thread somewhere on here about an old school lube tune that should have lots of info that would help.
I wonder if you were to wipe the oil off the leather washer and soak it in the neatsfoot oil, allow it to sink in then wipe off the excess, it will expand to form a better seal and fire with greater power?
Rich.