The sensors may be clean but the tone-ring can be dirty. They should not have to even touch the tone-ring when they turn the rotors....The tone ring is molded into the rotors and cannot be removed or shifted.
It is possible that the sensor bracket could have gotten bent just a little, enough to not be getting a good magnetic signal. I don't know the precise measurement, but they there is a specific air gap distance between the sensor and tone-ring. Also, be sure the tone-ring is clean.
Be sure to check the wiring to the sensors to be sure that they are not cut, chaffed, or broken. In the end it could just be a bad sensor? If everything else is OK, I would lean towards a bad sensor?
...Rich