[ujiro]

I had the same issues and it was what Kos said. V2 mechbox, and the little pin/protrusion that keeps the switch block (moving part of the switch assy) from going too far back was broken off. So, it made the trigger lock randomly, and then sometimes the trigger could move freely but wouldn't be pushing the switch block.

Unfortunately its a very annoying problem. Two options are, replace the mechbox shell, or drill a hole where the pin is supposed to be, tap it, and use a set screw.