[Oldsguy]

If it is like stamped steel valve covers, you can have a leak because the surface of the pan is dimpled at every bolt location and not flat, you don't get a good even compression so there is room for leaks around every bolt hole. You may have to remove the pan, flatten out the surface where the gasket mates to, install a new gasket and re-tighten the bolts to specs. As I said, this is just an idea based on my experience with valve covers.