It could be as simple as cleaning your antenna mast. I put a new mast in my SL last year and recently, it wouldn't raise all of the way. I gave the mast a good cleaning and it's working correctly again.
I also noticed that my relatively new mast is slightly bent towards the rear. I can only assume that this is from wind drag. So now, I leave the antenna button in the halfway up position. Stations still come in strong.
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino