I think you should put big rocks around the base of the
plant at least around the bottom where the water runs down. That will help to hold some water near the
plant instead of it running away down the hill. And put cedar mulch on them too. To hold moisture in. I always make sure I put mulch on new
plants just a safety precaution to keep moisture in. They might have had an insect on them that killed them. I am definately not an expert but I do have one steep hill where I have ground cover
roses. The soil is rocky and STEEP. I saw that the
roses were looking dry and wilted. I put rocks around the
plant to hold in water and mulch and they perked up and are doing fine now. I also put a female praying manthis on them to control any insects. I think praying manthis eats butterflies so I would not suggest that for your butterfly bushes.