Perhaps the plant is receiving too much water. Mine did this and I discovered that they develop root rot very easily. They need to be watered infrequently once established. In fact, I hardly ever water mine. Just when we have not had much rain. They also need a fast draining soil.
I just planted 4 of these in my West Virginia garden. I wanted a single-flowered variety because I've heard they attract hummingbirds, butterflies & bees. I ended up with these double-flowered bushes. The blooms are beautiful. Just wondering if I will be getting hummers, butterflies or bees on these also. It seems that the old-fashioned single flowered variety of anything attracts more nectar-lovers. Will those of you with these double-flowered blooms let me know your experience? Thanks!