I have a yucca that's not doing very well and I'm having trouble diagnosing the problem. I got it sometime in the fall, and left it in the nursery pot. I watered it once about a month or two after I brought it home, and the yellowing/wilting started then. I've trimmed back the wilting leaves and left it to dry out but the remaining leaves are shriveling and drooping. There's sand on top to deter gnats.