My guess would also be the piston. Inspect it whether it's not worn out. If it is, there's your problem. If it's not worn, then like Crusader mentioned, you could have the wrong type of piston (the catch on the piston is not compatible with your trigger box, e.g. 45 degree and 90 degree, may not work together.
Compare the new trigger box to the old one, how do the sears look?
I would look it up, but I will try to give you my best: The two trigger systems differ in the piston sears, the part of the trigger that grabs the piston and lets it go once you pull the lever.
A 90 deg sear is used on most L96 systems, and is generally the better choice.
This system is exactly what you would expect from a system used to hold back a piston: the sear goes straight up, 90 degrees from (perpendicular to) the cylinder and grabs a lip on the piston that is also perpendicular to the cylinder.
A 45 deg system uses a sear that's designed to be a flat plate at 45 degrees-facing up into the back of the cylinder. The sear grabs a lip on the piston that fits right onto the plate.
This system is more problematic because it introduces more forces to the trigger, piston and cylinder and is more prone to wear than a 90 deg setup. These are used on VSR's and M24's.
Both can work well, but ultimately whichever one you pick will depend on the availability for your gun and/or your machining ability. You MUST make sure, however, that you have compatible piston/trigger mech; they must both be the same system.
If you look at the link you gave us for your current trigger mech, you'll notice it's a 90 deg system because the piston sear is perpendicular to the top of the trigger box (and thus the cylinder) where it will grab the piston. This means the parts available for your gun will mostly be 90 deg parts. It looks like you will be getting a new piston.
If it was up to me, I would get an upgraded metal trigger box for any cylinder upgrades you get just to prevent any problems. Then worry about the bolt system. The UTG upgrade kit should work, but you may eventually have to worry about the spring width. Stock rifles usually come with smaller spring guides and holes in the spring coil (5 or 7mm, I forget) and upgraded power springs will be the larger thickness (7 or 9mm), so your spring guide size will matter.
Lastly, it looks like your gun is mostly compatible with L96 internals, so look for that label. I can't guarantee compatibility or every detail I put on here, but I did my best to give you some direction.
Ok thanks and I discover a small piece of metal keeping the piston from moving all the way back so I removed it and the gun works! So in a way it was the piston. Now that i have shot it and realized that the spring had been compressed for a while I think that I will upgrade the spring, spring guide, and probably the piston. Any recommendations?