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If you're sure all the doors are latched and closed tightly, then it sounds like one of the switches is malfunctioning. Starting with the easiest test, try pressing the switch on the leading edge of each rear door. If that has no effect, remove the screw from each switch, one at a time:
If that also has no effect, test the switches in the rear latches next. The liftgate has a single, central latch; the tailgate has one on each side.
If the switches are functioning normally, it's possible a connector has become undone. That will require removing some garnish to check.