I don't know if this will help at all, but since the system is vacuum operated it need help. i have heard that once you shut the car off there is only enough vacuum to operate the central locking system 7-9 times. After that you would have to restart the car. Maybe you have a small leak somewhere or your pump is vacuum starting to fail and not creating as much or as strong a vacuum as you need

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired