Perhaps, starting life in cutting rather than seedling form, gave them root systems that went down for water less vigorously. If true, this will be a tough problem to overcome. In fairness, my soil conditions are drier than most. Not only is the soil sandy but it contains fine sand grains. When the soil dries out it becomes like powder and, as pointed out above, is difficult to moisten.
Humus does make that moistening process easier.
With the exception of E. calorhabdos and E.macdonnellii, all plants here were grown from cuttings. The planting of seedlings was more successful, I believe, because they could send down long roots while young. Cutting-grown plants are now being watered in winter to encourage such root growth. They flower for many months if given water. They are not worried by either our dry or hot conditions provided they can form a deep root system.