Why is the trunk closing assistant not working in a Mercedes S500?

There is not one simple answer. There are a multitude of reasons your trunk closing assist does not work. In the MB S500 the doors are close assisted and they operate on the same system as the trunk. Do the door close assists work and the trunk not? Broad questions get broad answers. You can go to the related link to learn much about how the closing assist works on the S series of MB.