try cleaning up the valve cover where the gasket lies until its shiny and
spotless. Then glue the gasket to the cover with some high temp silicone
stuff. Don't use much and before it cures, offer (as the British say) up
the cover to the clean head and hold in position until the wire bail is
secure. The wire bail tends to move the cover just when it is snapped into
position. 
You checked to make sure the cover is not bent or dented?

Alistair