Hope some of the pros here can tell me where I messed up !!

My 300CD (1985) was leaking oil and my driveway was looking really bad, so I decided to buy a valve cover gasket at discount autoparts and take care of the problem.

Did the whole thing yesterday in one hour or so, and after a quick drive everything seemed fine.

Today after taking my wife to work I noticed a bigger oil stain on my driveway so i decided to check the engine just to find that there is oil still coming from the top of the engine through the gasket.

I tried Tightening the 4 bolts more but I'm afraid to mess them up.

Please can somebody give a clue of what could be wrong? I checked all around and it seems to be tight against the engine and i installed it with a lot of care and making sure no dust or debris got between the gasket and the metal, but since it was the first time I did it I may somehow messed up.

Thanks a million!!