The rocker cover gasket being a bit flimsey can be a bit tricky to sit in exactly the right position on the lengths without curving inwards. A simple solution is to fix it first to the the cover with gasket goo. Place it face down on a flat surface. When semi-set then fix the cover to the cylinder head.
This also has the advantage that if and when the cover is removed the gasket should come off with the cover and usually re useable.