[DoctorBadVibez]

I'm not an official gun doc on ASC so please consider that before following these steps. While I can only offer advice, contacting your local ASC Gun Doc maybe your only course of action if any of this seems complicated or headache forming (go aleve). I am merely suggesting different scenarios other than what others have posted, to further compliment the input you're getting.
This is all based on personal experiences with fixing the guns that I own, those that other people give me the gracious oppurtunity to fix and my constant fixation with finding the perfect fps/range ratio.

Things that can go wrong:
When you replaced the piston, did you check to see if the piston head diameter matches the one you replaced? (Out of three guns with low fps that I fixed, one had this cheap piston and no way it could make consistent compression)
Ok, pistons good, what kind of packaging did the o-ring come in? If its a non airtight, rather dry instead, it may just be the o-ring itself, its shriveled up, put it in some light 100% ge silicone oil for a bit you might be able to gain back your fps. There should never be any non lubricated rubber other than the hop up rubber. (That saved a couple of good builds here and there packaging for o-rings need to get to industry standards).
Failing the abovementioned, did you change your cylinder? OR damage it (squishing it) My personal AEG did terrible with a bore up cylinder on a SR16 standard length 6.04 barrel, it did a LOT better with the standard marui cylinder swapped in. A squished cylinder, toss it.
There's a thread somewhere on cylinder size to barrel volume somewhere on this forum, its VERY informative.
Another scenario, spring.
If no go...
Gun doc it.