well, awhile back i had the same problem. all the "simple" corals like zoas and shrooms would not make it, yet anything else would thrive perfectly. so, i kind of did a run down of everything.
-light, too much or not enough
-water pars. is everything 100% spot on?
-ro water, is my tds good?
-temp, maybe raise or lower it?
-salinity, maybe raise or lower it?
-fish/inverts i own, could anyone threaten them?
-placement of zoas? too high or low in tank? are they getting enough light where they are?
-flow? too much, not enough?
-is the system established enough?
-zoa accumulation period, drip long enough and correct enviroment?
-are they all coming from the same LFS?
-where are these zoas coming from in the world?
-are you dosing anything in the system?
-are you directly touching the coral while putting it in?
-do you use any air fresheners near the tank?
-did you wash your hands or do anything before putting the coral into the tank?
there are alot of questions to ask to figure it out. personally, i found out in order to keep all my zoas thriving, my TDS HASSSS to be 0, my magnesium level was too high, and i had too much flow in the placements i tried in the tank...