What’s causing this die off?

It’s spring here in Vic and a range of my plants all seem to be struggling. This time of year they should be thriving like the plants around them. I repotted in winter with seasoned soil and fertilizer which they all initially loved. I also treat them with white oil for pests a couple times a year. photo 1 – rose, 2 and 3 – oleander, 4 – not sure, 5 – avo. Any help would be appreciated.