Smart cover is temperamental

[sui_page]

I just bought a Nillkin smart cover for my new Nexus. When I open the cover sometimes the screen will switch on and other times it won't. Would anyone know the reason for this?

[sui_page]

I believe it has something to do with lock screen timing out. When I open immediatelt after closing it, it's fine. However if I leave it for say a minute the screen won't turn on.