why won't my ignition switch return to the lock position ?

probably the mechanical portion of the switch itself.

Replaced key cylinder. Still will not go to lock position.

Shifter cable problem....most likely in the center console/shifter area. In my scenario, IF the cable were removed from the ignition lock housing(must be in ON position), the key returned to the Lock position every time. Cable installed, key only goes to the Accessory position.