One additional possibility is that the home switch is gummed up, and reacting slowly causing the X to move to the next marker pulse but instead it hits the end of travel.
Jog the X axis back quite far, start the homing sequence, press the X home switch down and hold it down. If the X does not stop in half an inch of travel, you may be missing the marker pulse. Be careful not to get any fingers pinched.
George
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)