Many varieties thrive in much lower light levels than we might expect. So too much light is the #1 issue IMO. In fact I'm going to mount one or two of mine on the vertical plane low in the tank, under 6 T5 bulbs....ver low light. There may be more light loving varieties but I haven't found them yet.
I think many frags are too small and are too delicate for some reason. Try to buy ones that show evidence of growth on the frag plug - avoid fresh cuts. Try specimens of the 3 species groups to see what ones work best for you.
Maybe try the Julian Sprung Hollywood Stunner Chalice variety .... Very attractive IMO, widely available in large colonies, very affordable and most importantly very hardy IME. Allow no contact with algaes, zoas, clove polyps etc and keep away from other corals. Keep them stable during the intro period and mount them as soon as you are confident they're doing well. Domt procrastinate. They don't seem to like falling into the sand as I believe this irritates them or let's some pathogen possibly take hold.
So there is your list to consider. Just my 2 cents. I too struggled with a few fresh cut mini 1" pizza slices in the beginning but I now have 4 "winner" colonies and another is coming along. I'm not yet confident enough to try my hand at the really expensive & crazy rare specimens just yet, but am approaching that day.
It looks like your skills and your mineral parameters are solid so just keep at it - you'll get it.
