Pull the fuse in the trunk for closing assist for a few moments, that will reset the pump. The pump has safeties that have pressure and time parameters, most of the time it will work for a while. The system defeats the circuit if it can not sucessfully close a door on the third try. I am on my 3rd pump (replaced two) it appears they are 5 year items. The last time MB checked the hoses for leaks found none, and replaced the pump for free. There is a lot of info on "BenzWorld" search "closing assist" most of the time its a replace the pump $600 or so. The pump sends pressure to the doors to close, after it sees a signal from the lock, after the close confirmatio it sends vacuum to reset the diaphram. You will usually heasr a hiss in the drivers side of the trunk as it resets. The pump is located on top of the rear wheel passenger side way back take the lining out of the fender and the rear
http://www.benzworld.org/
http://www.beckmanntechnologies.com/. sell rebuilts, be careful about ebay you could get one no better than what you have