Your sunroof is in need of lubrication. The metal tracks on each side of your sunroof opening need to be cleaned and lubed. Mercedes makes a special lubricant just for sunroofs that is the only thing I have found to work well. I'm sure you can get something similiar elsewhere but I just don't know what was in it that made it work so well. It was about the color and texture of vasoline. Try cleaning them and lubricating them with a light oil such as WD40 or graphite spray then cycle the sunroof a few times and see if it doesn't loosen up.