Deep water zoas are somewhat more finiky than your average zoa. Something at night might be bugging them ( i.e. peppermint shrimp, cleaner shrimp, sundial snail, zoa eating nudi's, unicid worms etc etc. ). Some deep water zoas like very little light and some like more light. Also alot of zoas melt after being fragged. I have lost several of my favorite zoa colonies after fragging them. Also when you frag corals they try to rebuild/repair themselves so they use up more nutrients. So there may be a drop in nutrient levels for a couple days or that particular corals imune system is very weak.
Try adding a drop of iodine once in a while as it seems to help with my zoas and mushrooms etc.
Some fish will aquire a taste for zoas later on too. Just when you think they are safe they eat your favorite coral.
Hope you find out what's wrong.
Cheers