I see you posted this on one of Facebook groups also, I only glanced at a few replies are comical.
It's a Specna arms right? Without fully knowing what spring is actually installed it's difficult to know how bad the problem is. You mentioned a M100 but the ship could be just saying that judging on the fps drop. They could have fitted the M120 or M110, plus different brands are different powers etc.
You say you've used it for 4 games, over what period of time? Probably loss of compression in the piston head, cylinder head, nozzle, bucking etc combined with the cheap Chinese spring losing its power. Could even be something as simple as a regrease and oring.
If still in warranty the store should have opened it up to check though, wouldn't have taken long.