Do they get direct sun? All of mine only got a little bit of morning sun; otherwise bright (but not direct) light. I also watered the backbulbs (not as frequently as my plants - probably half as often) just so that the media didn't go bone dry.
If the growths were rotting off at the base, then it would be overwatering. If they simply dried out and died, then it would be lack of water (which could be that the bark is too coarse). The mix I used was a mix of sizes.
