Did you dip your zoos before putting them in the tank? Zoas are susceptible to small nudibranchs. And when they irritate them, they will close. You can look closely to see if there are any on the colony. Also look for eggs.

For a dip here is a link to a thread at ReefCentral by MuchoReef, a friend of mine and a zoo guru. http://www.reefcentral.com/forums/sh...d.php?t=451720