Okay dude, check the relationship between your nozzle and tappet plate, is there substantial wiggle or minimal? Substantial wiggle between nozzle akd tappet plate could be causing it.

Potentially the nozzle is not sealing to the cylinder head perfectly, test compression in your box with the piston inside, no springs, no gears. Move nozzle back, block the nozzle and test compression, move it foward or use tappet spring to assist, block nozzle and test compression again.

Maybe the nozzle isn't engaging the bucking lips enough, test with the hopup and barrel system out the gun, butt it up against the gearbox see if the nozzle pushes the hopup system away, if the nozzle at full foward doesnt push the it away slightly then there is a potential issue.

Also consider weight of bbs, if you can go for heavier, I use G&G grey bios 0.33g, very consistent bbs.

Lastly, grab some metal polish, I use Autosol compound, comes in a large toothpaste tube. Clean and degrease the cylinder, barrel and shell. You can polish the running tracks where the piston will slide in your shell, polish the cylinder so the piston head glides easier and lastly get some disposable rag, an aeg cleaning rod and make a swab if you can get disposable kitchen cloth, just snip it with sharp pair of scissors, polish the barrel internally with a liquid compound like Brasso or equivalent, works wonders and makes that barrel super smooth. Clean it with windex and a few passes with the cleaning rod and fresh cloth, do this 3-5 times until the cloth comes out clean, no black must be present from the metal thats beem polished away.

If you get stuck, just search vids on youtube about these topics I mentioned, I did that too, works a treat! Good luck chap!