I have been trying to grow Convolvulus cneorum for a few years along the top of a rockery (maybe 40 separate plants). Without warning they start to change colour and start to die. This is quite random and I have some plants took 2 or more years to die this way and others have not died. I replaced the soil in areas where they died with fresh garden soil from my landscape supplies after digging out the existing soil to a depth of at least 400mm. Note the garden is only about 300mm wide with large sandstone rocks on one edge and lawn the other side separated by pine sleepers which would be at least 10 years old. I replaced 8 plants into the new soil which had been placed there at least a month earlier just 2 months ago (watering them in daily for the first week and then once a week) and today I notice that two plants are starting to wilt and die. I feel that I should replace all of these Convolvulus with something else but what? See attached photos. What could be causing this?