I wish I could help you, but I've never heard of this happening. You say there were no roots at all when you sifted the soil? That would mean that it wasn't growing roots in the soil it's been in. What kind of soil was it in? What I would say is that if it's been surviving on the small amounts of roots that it has, is it'll probably still continue to live. But of course you should figure out why it's not healthier, and fix the issue. I would treat it like any other plant that's been repotted, and work it back into the sun slowly. I use a fert called quick start when I repot something with few roots, don't know that superthrive does anything...but I don't use it, so I have no actual experience with it. good luck.