Still sound like a fungal root rot to me, but hard to tell. Got pics? I doubt the layer of sand is much of an issue. Most shrubs gets down into the sandy layer anyway.
I think intense sun can burn the leaves, but if the whole thing is wilting and dying back is sounds like overwatering to me. Have you fertilized them? Do you have them mulched with wood chips? That can keep them more consistently moist, but new chips can rob them of nutrients as they initially compost down.