Zygopetalum won't grow roots?

[Miri the Wildmage]

Hi everyone,
I bought a Zygopetalum around March, not sure what cultivar. At the time it had a beautiful root system despite the gross rotten medium it was in, and as it had two new growths forming I decided it would be safe to repot. Shortly after repotting most of the old roots died off, presumably just from the shock of repotting since I hadn't let it get too wet or dry. Fine, whatever. New roots soon. However, fast forward about 4 months and although one of the new growths has kept growing and is now almost as big as the rest of the leaves (the other new growth died, but it was kind of damaged when I got it anyway) it has so far failed to produce any roots, and the old bulbs are getting more wrinkled every day from supporting it. At what point in the growth cycle are zygopetalums supposed to produce new roots, and any ideas why mine hasn't? Thanks!
P. S. I've tried kelp, higher and lower concentrations of fertiliser etc. and the plant is potted in bark and sphagnum