Hi Glenn.
It could be the back plates as they tend to corrode and drop bits of rust between the sensor and the bearing, which then in turn can damage the built in magnetic ring, so worth getting the discs off, and the bearing to check the back plate once you have it in your hands.
then again it could just be the sensor, I recently changed one out for a local lad, had to drill it out in the end as it snapped off trying to pull it out.