Mine has it too as does my wife's 540. Part of the Comfort Access package.
Do you have the trailer hitch? Someone posted that you loose the feature with the hitch.
What I've found is if the key is "too close" to the rear of the vehicle, it won't open. This usually happens when the key is in my left pocket and I use my left leg ( or right and right). If I use the opposite leg it works.
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