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 =).