Mine doesn't work because the sensor is bad. It could also be that the wire to the sensor isn't connected, needs the contacts cleaned, or is broken somewhere.
You will find that on almost all quads nuetral lights dont work. This is due to the poor location of the wire connection to the sensor (on the side/bottom of the case).
Half the time it isnt even the sensor its the ****** thin wire and the plug they use. If you clean out the plug and the wire isnt broken it should work.