Have to set the sag/preload on the bike? The rear shock is adjustable, what setting is it on? Did you test/inspect the shock itself when you had the spring out?
Check out the videos on how to set the sag, and what suspension is.
https://youtu.be/ZtzTyCKh5fY
https://youtu.be/uz_p-iu9C_s
Here is a link to race tech, check out the calculator.as well.
https://racetech.com/VehicleSearch
Below is a picture of the NewGen rear shock installed in my PreGen 250.If you look at the preload adjuster it's set on the highest (stiffest) setting. If you need it softer, adjust the preload by turning it down.
Once again what was the sag on the bike?