Well on my Lancer Tac full metal pdw, I took it apart to replace some of the internals, while reassembling it the ambi fire selectors will not screw in, i put the tiny ball bearing under the main fire selector (left) and try to screw it in, it is almost like the screw isnt reaching the hole. It is not stripped if I remove the fire selector and try to just screw the screw in it works fine, same on the other side. Any reason why it wont go in? How much would this cost to fix because I know its a simple yet frustrating problem.... Money is kind of tight so a answer quick would be nice. Thanks!