oil was changed and now the oil life reset will not stay reset, why?

2006-2007 Mercury Mountaineer Turn the ignition to run (one position before starting engine). Enter SYSTEM CHECK to display “OIL XXX% OIL LIFE OK”. Press RESET button to display “HOLD RESET IF NEW OIL”. Press and hold the RESET button for more than two seconds. Oil life is set to 100% and “OIL CHANGE SET TO 100%” is displayed. Turn the ignition off and then start the engine to verify the indicator has been reset.