There's nothing wrong with the safety catch, it merely blocks the trigger, and it's not connected in any way to the anti-beartrap, which is where your problem lies.
Sounds a bit like the closed sidelever is not pressing the anti-beartrap release lever far enough in - check it's not been re-assembled incorrectly.. If you have to fire it in the way you described, it's a good idea to open the loading tap before doing so; it's a lot less violent and dangerous if you do. The trapped air will leak away fairly quickly, or you can gently close the tap a bit and the piston will slide forward safely.