The thin metal of the valve cover and galley cover can bend if you overtighten the screws. Once bent, the seal is compromised. Tightening the screws again just makes it worse. When the covers are bent, I take them off and straighten them to original shape on something like a flat bar. I put on a new cork gasket and don't overtighten. Some owners like to put sealant on the cover side of the gasket. You can also purchase load spreaders to apply more even pressure. The problem is that they are noticeable if you wish to maintain an original looking engine. To get cork to seal it needs to saturate with oil. It eventually will leak a very small amount of oil and turn black but this is normal.
