Mine doesn't even work. The selector moves but doesn't do anything, the things full auto all the time. I couldn't care less at the moment as I don't use semi anyway. Probably just a loose wire for me.
Can you move your selector at all? If you cant at all a wire might have moved a bit and jammed it stuck. Probably best to get someone experienced to do it for you, its probably buried into the gearbox, could be difficult if you've never done it before.
Insert Something Witty and Profound.
The fire selector is just connected to plate/control rod construction that, if I remember correctly, connects to the auto cut off lever. No need to disassemble the whole gun. If you take off the trigger guard you should see what the fire selector pushes on, try moving this plate manually. If it wont move then the control rod is probably not seated on the lever correctly, in which case you might have to take the gun apart to get it set. Bear in mind its a lot easier to take apart than reassemble, you have been warned 
