The problem more times than not is the gasket surface on the head itself. It isn't flat and straight like many assume and is, if you lay a straight edge on it length-wise, you will see it is arched most of the time making it difficult for a gasket to seal. And this is the cause of most valve cover leakage on these heads. This isn't too much of a big deal for a sheetmetal valve cover that can bend and flex some unless they get to warped and distorted, but for a more rigid valve cover that can't conform to the arch of the gasket surface as easily it is.
