knowing nothing about the pot or soil mix, only what the tree is doing, I'll take overwatering/not draining well for $500. You've picked one of the second most difficult/finicky trees to keep. The very first thing to remember about them is they DO NOT like wet feet at all. I will suspect that it is root bound and the pot is not draining or the soil is staying too wet too long (soggy).

They will grow pretty well in a mix of Turface or Haydite and medium Pine bark, just make sure that the pot drains well.