Check that with the sensor over the homing tab, the sensor consistently reads as triggered. You may need to stick a piece of white card or tape over the homing tab.
Also check what mode the sensor it in and what the trigger value (G31 P parameter) is. AFAIR, RepRapPro sometimes changed the probe mode (M558 P parameter) from 2 to 1 during X homing. I don't know why they did that.