If you're sure that the end of the long side of the sear spring is on top of the stop pin, and that the short end of the trigger spring is under the stop pin... (you may also need to twist the trigger spring around so that the loop is smaller to get tension???)...if the springs are fitted correctly, then I think it can only be a worn sear/trigger, or the slot in the piston that the sear engages with?...been ages so really just guessing