Regarding that rotor sensor with lugs, how thin are the lugs themselves? And is it the original item/what age is it?
I had this problem and while the sensor unit itself was fine and unbroken, the lugs were worn down and therefore not spinning in the unit. My sensor rotor was original (bike's an '51 reg) and it was just worn thin. POSSIBLY it may have been jiggled when you last had the wheel off and hence dislodged it slightly, thus causing it to wear down more than normal.
Just a thought!