If it's a pressed steel rocker cover Dave, with thick gasket these have a habit over the years of distorting around the screw down holes. This means it becomes increasingly difficult to get an even pressure on even a new cork gasket with further tightening in an attempt to stop leaks exacerbating the situation. Put rocker cover on flat surface and look to see how it sits. It may be possible to carefully tap back metal around holes with a support the other side, but take care as you may make the situation worse. Once done be careful not to overtighten although I admit you will probably need to use non-hardening sealant as well.
