[NWPilgrim]

Sounds like how my Inland Carbine behaves with underpowered reloads. Since you are using commercial ammo then it is probably something that causes it to have lower power on the piston operation: gas port plugged by crud (soak barrel with solvent); piston/cylinder carbon crudded; operating rod rubbing somewhere or gunked up with thick cosmoline/crud; bolt or receiver crudded up or has thick dried oil/cosmoline.

I doubt it is the spring. I would guess a thorough cleaning and lube of barrel, gas port, piston and cylinder, op rod and recoil spring, bolt and receiver will solve the problem.