You sound like you 1/2 way know how to put valve cover gaskets on. Every cover leak I ever had the gasket had slipped out or you could see a reason for it. If after about the 3 rd. time it was time to look for something else. There is no pressure or standing oil in the cover so it shouldn't be hard to seal. I'd be looking for stopped up return holes, crack in the cover or a stopped up breather.