My Myoporums get dieing

I live out here in the Phoenix area and I have about 95 Myoporums to cove the gravel which is mandatory where I live. I don't know if I'm over or under watering them or planting them in a big enough hole with enough mulch because the ground is rocky. But I'm losing about 25 to 30 % of them and I just keep re-placing them. Anyone has as ideas how I should treat them this time of year?
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