If you listen carefuilly (engine off, ignition on) you should be able to hear a "click" from the front brake microswitch. If you can't hear a click, the switch is misplaced/missing/broken.
They can become displaced. Mine has two tiny lugs which locate in two holes in the handlebar switch/master cylinder assembly, although I believe there are two different types. Pull the switch out, give it a squirt of WD 40, test that it works by pushing the button, and carefully reinstate it.
Threaded Mode