FX Velocity won’t increase.

I recently bought a used WC mk3 22 cal. Trying to shoot fx atomic 15.9grn. I can’t get the WC to shoot past 800fps. I’ve increased the reg from 80 to 150 and adj hammer accordingly. Also swapped from 500 to 600mm…no change. Also put a power plenum on and still no change. I switch the 600mm from pellet to slug and still no change. Any suggestions what it could be? Thanks
 
The .22s were mostly stock with a 7 gram weight. The 11 was in the .25. That being said, I bought the 11 gram for both of mine to give a wider range for the reg. One other thing to check would be to make sure the cocking arm link has not slipped. If it has a stock probe in particular it could be covering the transfer port.
They changed this to 11 gram weight before the Maverick came out. 11 is standard now.