Yeah i thibk when i first got mine they didnt open much and i bought another one that was shipped roughed that nevee did open up
But i did put them in differnt tank and they didnt open up much and when they make babies they close up a little bit more but no i dont know why yours wouldnt was just giving some back ground on mine. If was me personally id try get nutrients up more and move them around in tank till they are happy but if you got other zoas in tank that are happy then its probably not a nutrient problem u got a pic of them?
Also do u know if they were recently fragged maybe?