Gordon posted "But I would suspect the flatness of your current cover mating surface."
This I can totally agree with. I've had aftermarket chrome and cast aluminum valve covers on V-8's not seal well, allowing oil seepage or outright leaks and the most likely cause has been the the valve covers have been slightly warped and not sitting true to the mating surface. This has been more prevalent with Chrome steel covers in my experience, though I have had a pair of aluminum that were really tweaked. Also if using a chromed valve cover it often helps to gently sand off the chrome plating on the mating surface, for some reason when plated the uniformity of the plating is not uniform on the mating surface, and doing the same to an aluminum cover has proven to be beneficial as often the casting on the mating surface is not all that smooth .