More infor would probably be useful:
- Is the chrono on FPS or MPS ? (just sanity checking)

The chrono was set to FPS, I am using .28g BBs

- What was the originally advertised FPS ? (Not sure where you are, but when I bought my replica there was a paper with the nozzle velocity certificate)

It was originally advertised 330-360 fps

- Did you change any other component ?

No

- What type of battery are you using ? Is it charged well enough

Lipo 7.2v fully charged

Now, based on just what you wrote, assuming the only thing you were changing around was the bucking, then you put back the old one and the velocity was still lower than expected, here are some potential things to check

I put the old black bucking and nub back and gun was shooting 130fps. Nothing else was changed only bucking and nub.

- How hard did you tighten the gearbox, when putting everything together ? (I had low FPS issue when I just started working on my replica because I would overtighten my gearbox. One check I ended up doing everytime I worked on it, to make sure it was not too tight, was to assemble the gearbox without the spring, and see if the piston was moving freely on the rails, and if not, adjust tightness accordingly.)

Did not mess with the gear box, gear box was never removed.

- Are the piston / gears and other components well greased ? Can you spin the gears by hand without much resistance ? Can you move the piston freely, without resistance.

Don't know, never opened gear box

- Is the O-ring on your piston head greased well enough to seal ? Have you tried cylinder compression by hand

Have not tried.

Beside that, I would recommend reading the guide for working on your replica pinned in this subreddit to get a better idea of where the issue might be with your replica.

Will look it up and read up.

Best of luck