[mcguyver]

The problem may lie with your switch. I've seen a few guns cross my bench, both stock and upgraded, where fouling of the switch contacts has occured. This is usually from lubricants in the mechbox becoming deposite on the switch contacts.

During firing, the switch is closed when you pull the trigger. All the current necessary to run the gun passes through the switch. This current will heat up the deposits on the contacts and cause arcing across the contacts. Repeated arcing can form a black slag of oxidized metal and deposits and can have enough resistance to impede current flow. The fix is to remove any deposits and file down the burnt contacts until the are smooth and clean.

There are also contacts at the selector between the switch and the selector plate. During press fitting of the contact plate on the selector at the factory, small burrs of plastic can be left on the selector plate. These have been known to push the lower switch contact arm out of the way when the selector is moved to semi or full auto. This will also break the circuit and lead to a no-fire situation. Removal of the burrs off the selector will solve this problem. I have only seen this on Marui guns.