M4 safety lever problem

[Wutang]

I have a really weird problem involving the safety catch/lever. When I put the gearbox together without it, it will shoot both semi and full auto fine. When I install it, the gun will fire full auto normally, but when it is switched to semi, it will fire only once. Once fired on semi once, the gun will no longer fire in any mode, like the battery got disconnected. The trigger pull is also lighter, like the trigger is on longer pushing the wiring loom into the trigger contacts. However, when the gun is then turned to safe and the trigger is pulled as far as it can go, eventually there will be a clicking noise, like a spring snapping back into place, and then the gun will fire on semi once and then die again. This is not my first time inside an AEG gearbox or installing the safety parts, what can the issue be? Thanks a lot.