Not quite sure.

I thought I cleaned out the hole quite well before putting the sensor back in, but I don't think I greased the o ring and that could have binded with the surface when pulling it out?? I do vaguely remember having to slightly persuade the sensor to go into the hole and I thought it was just the o ring binding, however, when the casing was removed it looked like it was more bullet shaped than flat so I am wondering if my tapping pushed the magnet down??

I didn't actually see it since my helper did and threw it away before I had a chance to see it. When I showed him the new sensor he was wondering why it looked "different" from the old sensor. That is when he told me that the old sensor looked bullet shaped.

Needless to say, I cleaned the hole thoroughly with a stainless steel pipe brush that cleaned all of the detritus from the inside of the hole. I will put some silicone grease on the o ring.