First off, I would check the piston end for wear. Secondly the sears, that they settle properly to the mech. After you've checked thoes, Check that the cylinder is fitted properly and snug in the receiver. Also the culprit might be loose screws or high tolerances, Tighten the two screws that attach the triger mech to the receiver tightly.
If any of thoes steps on on finding the culprit don't solve the problem, Check the Trigger itself for wear. It is possible that it has worn, so that the first sear sets plase too low in the mech, and it will not catch the piston. I had this problem on my rifle after running my vsr at 550fps/0,2gbb for 2 years.
Note that the Pro- and the Gspec- triggers are a bit different on their design, so if you have to replace the trigger, you can choose the one you like more, I personally prefer the Gspec model, 'cause of the low profile design and less grooved shape on it =).
It can be cracking, rounding off of the top edge etc basicaly anything that looks like it would cause the piston to release early.
Couple of things nakkis missed....
Your trigger sear might be stiff or the spring might be broken, fitted backwards and, as in one case I've seen when someone fitted new sears to their weapon, not even there, in a lost it type of fashion :-/.
To check this remove your trigger group and you should see the piston sear sticking up, make sure it returns to its full height when you push it down without any fouling or sticking. It should 'twang', for want of a better description, back up when flicked with your finger nail.
The other thing to check is the actual sear adjustment.
Through the holes in the side of the trigger group you will see the sears interacting with one another. Make sure that when they are under spring tension they aren't set so that they spring apart straight away. You adjust them useing the 2 allen screws near the trigger itself.
I think i fond the problem! I was just working on the gun when i looked at the piston (like you guys said). Apparently, the sears are to hard for the piston, because it was just stripping away the plastic, smoothing out the edges. Now my question is, if i upgrade to a metal piston, do i have to use a metal spring guide to? Also, are the airsoft gi pe parts any good, i used their barrel, and it works pretty well. The reason i am asking is because i am not a rich airsofter and cost is an issue. If it wasn't, i would have already gotten the other polar star parts.