back off your power adjuster a couple turns and shoot it at 2000psi and see if the fps doesn't go up.
Your hammer may be binding on the inside of the tube and causing it to hit the valve with a weaker amount of pressure???
My .22 disco shoots 14.3 CPHP's at 960 with the adjuster turned in.
If that doesn't work, try the tissue or talcum powder test on your breech seal.
Also, run some orange glow citrus based cleaner down your barrel and give it a good cleaning. It could be gunked up.
Just some things to think about.