Well, I've been at it almost 30 years now and actively growing corals thru most of that. I've used every lighting system and experimented with multiple approaches. I now have it fine tuned to the extent I have a pretty good picture of what some (certainly not all) different corals like and dislike.
But I have found, as you have, some corals, especially frags can die right next to one almost identical and it is growing gang busters. I have had plugs that just sat there, didn't die but didn't grow an inch for a year, then suddenly took off. I have had what were considered very hard SPS
corals growing like crazy while a common Capricornus next to it is dying. I have some friends like mr_X that have a "wet green thumb" and without excess worry seems to grow corals just fine. I am an engineer and am always trying to get at the bottom of stuff, but with this I have determined all you can do is buy good established frags then cross your fingers.