Struggling with Double Delight — would appreciate advice

I want to be honest: I'm not getting great results with Double Delight and I'm not entirely sure why. powdery mildew in humid summers has been a recurring problem despite my attempts to address it. I've been focusing on deep watering at the base rather than overhead based on everything I've read.

exceptional fragrance on warm afternoons is there in glimpses, which tells me the plant has potential in my conditions. I have it in a sunny south-facing garden border which might not be ideal. peak flowering through midsummer showed some improvement. Two stars for now — not the plant's fault, but not a success yet. I'll update if things improve.