Zoo are funny ,make sure you dont have anything bothering them ,try to move them to a diff palce in your tank see if that works .
what lighting are you using ,what size tank ,flow , do you have any fish that might be nipping on them maybe crabs that come out at night .
I have some zoos that deside they dont want to open up and look like they are melting away just to see them fully open a week later .I use to think they are a easy coral to keep ,but they have so many freaking secrets i cant figure them out any more .
Try to put more flow on them but not too much .
good luck
Ronen