Dogpack
I've had clivia go like that and it usually was from root rot. When I first started growing them I put them in ordinary commercial potting soil which stayed wet much longer than they like. I now grow them in chunks of coco fiber with some shredded coco fiber as well. The pot drains well and dries a bit between waterings though I do have to fertilize it more often during the growing season. Last year I found out they like a cool dry period in the winter with just enough water to keep them from shrivelling. They were in a 50 deg. F. room and rewarded me this past spring with LARGE tall spikes and lots of new growth. I would take yours out of their pots and check the roots. If they aren't thick white and firm, I'd change your potting medium for one that drains quickly, cut off all the black mushy roots, dust the end with a bit of horticultural sulphur or another fungicide and replant in the new medium. Don't water much until you see ( pull aside some medium) healthy new roots. Also when you water try not to get water in the crown. This was my experience but I'm no expert on clivia. Since I've switched to the coco fiber my clivia seedlings have really grown a lot better too, this year they more than doubled their size from the previous two years combined. Good Luck!
