You have a broken trigger spring. When this tiny spring breaks, the trigger won’t reset. When you cock the gun, the piston does not catch the sear because the sear is not in its proper position. I have had several do this. Remove the trigger unit and you will see one of the loops on the end of the spring has broken off. I just remove the spring and with tiny needle nose pliers and remake a new loop. Easier than trying to get one.