Help! New Trichocentrum tigrinum with no roots

[Svham]

I got one of these off ebay about four weeks ago. It has three healthy older growths, and one healthy new growth. It was potted in a pretty broken down looking bark mix, and when I took it out to look at the roots it was bad news. It had NO live roots that I could see, and I looked pretty hard. I soaked it in some superthrive and re-potted it in a clay pot with some sphagnum and med/lg sponge rock. It has been in a bright south facing window, and gets watered when it is just starting to dry out - about once or twice a week. I decided to check and see how it was doing today, and when I popped it out of the pot it didn't look any better. No worse, but not even the tiniest start of any new root growth. The top growth still looks pretty healthy.

Any ideas on what I should be doing with this guy?