Your vehicle is equipped with a PSE (pnuematic system equipment) pump which has a regulating block which controls all of the vehicles vacuum assist features. This complaint can be caused usually by one of two things. First a vacuum leak. Do you hear anything that sounds like a vacuum leak in the door? A hissing sound when the closing function is operating? If you do you have a broken vacuum line to the closing assist actuater. If you don't here a hiss the valve for this door may be leaking at the PSE pump. This would require replacement of the entire pump as the distribution block for all functions is contained within the pump unit.