So that's what I'm doing wrong with my truncata, I'm watering it.
The mix actually holds water for a couple days but if you miss watering it the plant will suffer. I put a saucer underneath to hold a little water so I wouldn't have to be so precise when I water. It wouldn't hurt to water every day with this mix.
I water every day with my current mix of cedar bark/mulch, pumice and orchid bark except when it is cloudy and cool.