[SHÖCK]

This is a common issue with G36s. They will fire when it's just the gearbox but once you seat it into the grip and the upper reciever, it will stop firing. It's likely a small pinch somewhere in the grip area or above the trigger or in your wiring. The screw on the plate that puts the gun into safe mode latch (not sure what this is called) might also be too tight, causing the trigger to click. Another issue I encountered was that sometimes the trigger didn't move far back enough because once into the grip, the slot for it wasn't deep enough so I shaved some of the plastic at the back of the trigger slot. That allowed the trigger to move more fully and the contacts to make better connection.