Sig 552 not feeding reliably

[AxisOfOil]

Okay... my project gun... a JG Sig 552.

Version 3 gearbox

When I fire it, the gun cycles... and it sounds like the gun is firing regularly.
However, it will only fire about every 5th-6th time the piston cycles... with the gearbox out, it looks like the nozzle isn't cycling the same number of cycles as the gearbox (i.e. every 5 times the piston fires, the nozzle moves back and forth 3 times) or something like that. It also has a severely lowered fps (160-250 instead of 410 like it used to be) (i suspect this is from the same problem) when it does fire.

Any idea what's going on?

I've tried:
Opening the gearbox, the tappet plate is not damaged, and the sector gear is not damaged.
Changing the piston/piston head/spring
Crying in the corner


Any suggestions, tips, insider information, new ways to threaten the gun into firing?
... I want to go to the MTT game, but it's my only gun that fires under the FPS limits for them... :'(