There are lots of variables causing AEGs low FPS issues. The air seal is a key point! The hop-up spring, though small, accomplishes big as it can pushes the hop up chamber back into the air nozzle to reach a good air seal. When the hop up spring is not strong enough or too short, or you lost it when you rebuild your AEGs, the hop up unit would moves back and forth, so it doesn’t always line up with the magazine, with the hop up moving it may cause loss of air seal, causing lower muzzle velocity.