I had this problem on my 4.8is as well. It has to do with voltage. At least in my case. From your picture, I am assuming you have a 04-06 4.4. When I would unlock, the drivers would unlock, hit the button again, just the front pass would unlock, but not the rear. Even the central button in the center console wouldn't work. Cycle it a few times and then it unlocks. I bet if it happens, you can hear the actuator working the rear doors. But when it is about to happen, you don't hear the click.

After hooking up my car to GT-1, I found out I was getting inconsistant voltage. I changed out the Voltage reg... the problem has never come back.

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2011 E70 50i M Sport