I actually have this same problem. It's only on my nerve with the LPR extender, so this is my theory...(Short story as well)..
Couple weeks ago went to play with gun after adding LPR extender. Wouldn't shoot, figured the solenoid actually went bad and used my other non-lpr gun for the day. Get home, disassemble, compare parts.
Non-LPR gun has ball bearing that the spring in the lpr extender under the adjustment cap sits on. Springs are the same length in both guns. I add the bearing to the LPR extender gun, it shoots. Take it out, no shot. Even with the adjustment turned all the way in. So I add the ball bearing, dry fire once, works, and throw the gun in my bag to go shoot it the next time I'm at the field.
Goto the field today, bring the LPR extended gun, chrono it, and I can't get it above 270ish fps. Mostly in the 240-260 range. Turn up the reg to 220, adjust the lpr endcap, turn it in all the way, dwell on 30, and the highest I get is 270. Turn the reg up to 240-260, and I still can't break 270fps. Back it down to 220 and play for the day. Rainbowing all day, but oh well. Everyone with the LPR on the nerve says they shoot 300fps with 180-210 psi at the verticle reg, so here's my theory...
SUPPOSEDLY there's a longer spring that they give you to put under the adjustment knob on the LPR when you add the extender. This would make sense because I simply swapped internals and therefore my spring is too short. The ball bearing gets it just enough extra length to push in the poppet and fire, but not enough to get the airflow required for 300fps, even at 260 psi. So try and find a longer spring is apparently what's needed, or at least that's what I'm thinking now. I hope that this all makes sense, let me know what you find out as well!