I have an oncid that was having major problems. I repotted it into a very airy open clay pot, placed a layer of coconut fiber in the bottom, placed a small amount of fir bark into the middle, placed the oncid on top of that and then added coconut fiber to the top and it has really taken off. There is a new growth coming along fast and the roots are growing nicely. They just needed way more air movement. I'm guessing bark holds the water too long and the thinner roots need to dry out faster.
Hope this helps