I had them in my very dry, sandy woods at my old place.

Woodland conditions.

I had one nice clump at my other house that thrived; planted lots more and none of them lasted. I had extra roots (ordered in bulk) so gave some to my next door neighbor -- they grew like weeds over there but mine disappeared except that one original big clump. I think it might have been voles or some other creature having at the roots -- she had cats that she kept outdoors much of the time, so that's the explanation that made the most sense to me. Because literally 40 feet away she had a lush carpet of them and I had zilch (well, not zilch -- ONE).

I took that one clump with me when I moved and it's now well established. Also added more and they're all doing well and spreading. The conditions I have them in here are moist soil, part shade for the quickly expanding patch, and semi-dry soil, part shade for that one big clump -- it isn't spreading, but it is a big healthy clump. So I think moister soil encourages expansion.

Not like a rain garden type area, right? I have a low area in shade, but I hesitate to plant things there because it does flood at times.

One shade native that I know for sure tolerates occasional flooding are bellworts--Uvularia grandifolia.