Are you making sure to dip when you get these corals?
Also, IME, wild colonies really need slow acclimation. They take a lot longer to become acustomed to their new surrounding environment. I would give it a try again but leave it lower in the tank for longer. Make sure to dip.
I don't know that I would frag that colony just yet, I'd leave it for a couple months to see if it rebounds. I had a colony completely go white on me, it retracted it's tissue and shedded some, algea grew over it, then all of a sudden it started coming back and the algea fell off.