Dead Westringia?

Confused as I keep losing Westringia glabras. One week they have new green growth then suddenly turn brown. When pulled out they seem dry but I am watering them deeply once a week. Was told by the nursery that they like drying out between watering but this is too dry!? Have been in the ground for about 5-6 months, giving them a prune a few times for shape. Located in Canberra.