FPS drop on my l96?

I have a heavily upgraded utg l96, sears and metal triggerbox, a angel custom piston and piston head, a prommy 6.03 barrel, pdi hop up, scs nub, systema bucking, and the spring and spring guide from the utg upgrade kit, I recently put in the piston and piston head, and now I'm noticing an fps drop, it should be shooting 550 with .2, and with .4s I was able to go through almost 4 cans on a poor mans chrono. I just did a poor mans chrono after having the piston and piston head in for about 2 weeks, and I go through 1 can and dent the second, thats a major drop, and I don't know why? Is it the piston head? I have to be losing air somewhere, my best head is the piston head because I was reading recently that it can change shape with use and cause a bad air seal, but thats only a guess, it still looks normal shape to me but I'm not sure.