Sometimes a little dirt or dust on the Hall sensor can screw up the operation of the feedback network. Been there. Done that. Fixed that by just a careful cleaning. Of course, if you did anything to alter the spatial relation between the sensor and the magnet, that too would or could cause the problem you describe. It's rather critical that the two pass within less than a very tiny distance of each other (max distance between the two is mentioned in the instructions but I forget exactly).