I'm by no means an expert, but It could be a few things...

Are the roots drying out, or are they dying back? Drying out may suggest that either you're not watering enough or the humidity is too low. What kind of mount is it? You said palm was used prior. Palm seems to hold on to more water than say Grapewood would.
If they are dying back, it might be the sap content in the wood, or salts from fertilization. Different plants prefer different environments. I found my dancing lady was dying back because of the combination of fertilizer with the rain water, even though it was less than a 1/4 solution of the recommended level. I never did an analysis on the combination but when I stopped fertilizing the problem stopped and they are growing like crazy now.