Did you use the correct MB sealant for this purpose. The correct one is translucent blue and smells more like plastic than the acetic acid smell of silicone seal. This sealant is very slippery and allows you to move the camshaft rearward while applying downward pressure without disturbing the bottom U seal.
You must clean the groove for the U seal thoroughly so that the vertical section will fit into the groove and hold in place. Clean the entire gasket sealing area on the lower cover, head and upper cover THOROUGHLY. Put the magic sealant everywhere, and gob it in the lower corners. Oil the shaft seal and carefully put it in place over the camshaft end with your fingernail while moving the cover rearward.
Mine leaked for a long time before having to remove it for other reasons. The leak was very minor, minor enough that it was just no problem in and of itself, but once I learned how to put the cover in place and what sealant to use, I found it no problem to seal it properly.
Best of luck,