Since this is a used gun (??) It may be the cursed "previous owner's" attempts to reduce the trigger pull. The hammer spring may be the culprit but test the gun again at the range and be sure the bolt is fully closed with every shot. If you still have light strikes clean the barrel, inspecting for a buildup of lead just in front of the chamber inside the barrel. The chamber area might also be dirty resisting the entrance of the feeding cartridge, leaving the bolt slightly open such that the cartridge is not fully seated. Also look for a firing pin burr on the breech face of the barrel. That too can resist the cartridge's effort to seat in the chamber. Do not remove the burr physically but buy the Browned tool to iron the burr back in place to fill the hole left by the firing pin hammering on the breech face from dry firing.
It might also be the magazine. Theses magazines are plastic and the lips subject to wear or even distortion. If they misguide the rounds such that they bump into the edge of the chamber so hard that the bullet nose is distorted then the feeding cartridge may not be able to fully seat into the chamber, holding the bolt slightly open, and forcing the hammer strike to fully seat the cartridge. That robs energy from the firing pin strike...hence, light strikes. Try a new magazine.
Just to be clear on nomenclature, I have no idea what SIG calls the various springs in the gun but "Main Spring" is not universally understood. There is the hammer spring, the recoil spring, and the sear spring. Which is the main spring? I think you were referring to the hammer spring by my nomenclature. But in this case either spring, recoil of hammer, might be the culprit if either was changed by the previous owner. I suggest changing both for new correct springs if the above information does not solve your problem.
LDBennett