Sally, did you unpot it and check the roots? There may be a problem there. I would take a few better leaves and try to root them. This way it's not a total loss. I've had that problem before. I had Splendeferous wicked on a grid and thought it was doing great. All of a sudden I noticed it was limp. When I took it out of the pot, all the roots were gone, and soil was dry. It looks like the wick stopped working and I didn't notice. It was in the back of the tray and easily overlooked. Now I try to check each tray to make sure plants are OK. and getting water.
I would agree with Lynn. Looks like a light or temperature problem more than anything else. Well, and some plants sometimes just want to throw a tantrum...
Very rarely have I heard of cases where certain plants or varieties just didn't want to thrive in S/H and I suspect that the plants were simply kept too wet or too dry rather than blaming the growing method/medium.
Years back, I used to grow most of my succulents in S/H and I have seen spectacular specimen of Streptocarpus and a host of other plants grown that way. Even some of the notoriously fussy houseplants like rex Begonias or some of the Marantaceae or Araceae can easily be grown to specimen size on a window sill using this method. S/H is pretty popular in Europe and has been for quite some time.