[Ratters]

Have you checked to see if the selector plate contacts are the issue? Sometimes they are not flush up enough and can cause issues making a positive contact to the trigger unit. Heat will also make things expand. A volt meter could also be handy to check continuity.

I'm assuming it's a standard V2 selector bar? If it's an Ares style push button trigger SWITCH, "click-click" I would try another one first in case the first one was faulty.