The spring's breaking in, as all springs do. Hurricane was right, higher springs drop more fps than lower springs when they break in. Maybe it's a fixed percentage of the spring's potential energy that's lost over time, I don't know.
There's 9mm and 13mm springs/pistons/spring guides. 13mm is better. Sounds like you have a 9mm spring on a 9mm spring guide with a 13mm piston. The spring, spring guide, and piston should all be pretty tight against each other.
Real Sword SVD and KWA MP7. Accuracy by precision or volume, the choice is mine.
Own it, paint it, mod it, love it