Persistent leaky valve cover on LA

Are the covers hitting the intake? Sometimes that will make them leak.
This is how I do it. I only use the stock cork ones. the flanges and head surface have to be 100% clean of all grease and oil, including from your skin. Use a small amount of weatherstrip adhesive to glue the gasket to the tin cover. Place the cover in place, and start all the bolts by hand. Then, using a 1/4" drive wratchet and extension so you are really only using your wrist to torque them, start with the center bolt on the bottom. Snug it down but not fully. Then do the two lower corner ones. Then snug all three lower bolts up. Then do the two top bolts the same way. Snug them both, then tighten using only your wrist and the small wratchet. If the cover bends, you went too far. Doing them this way for 35 years - rarely a leak. Plus I've reused the same gaskets for years doing lash adjustments.
Three main causes for leaks: oil or grease on the mating surfaces, the gasket is too stiff, or the fasteners were overtightened.