First, make sure the pin is in. On the top of the gun, right above the trigger, is a small hole with a set screw in it. That screw is the trigger adjustment. Its probably turned out too far and is holding the cap in place. Find the right size allen key and turn the screw down inside so it dosen't hit the tube.
Its also possible that the spring is either compressed or collapsed inside the piston. If it has failed and either broken or collapsed it won't push the cap out in any case. Will the cap come off if you take out the pin?
What spring are you putting in it? The smaller Vulcan-based actions like the Stingray and Xocet don't really respond well to more powerful springs. They are at about their limit as they come, and more spring just overpowers the seal and makes firing cycle harsh. A little less spring makes the gun shoot better in most cases.
