My Kousa dogwood was planted 6-8 years ago. It blooms profusely in mid-late June but, come July, the leaves start to show signs of stress -- droopy with wavy edges. The soil is sandy; could it be not getting enough water? Although we have had a couple of heavy rains, they have been a week apart or more. We do not provide additional water.
Kousa Dogwood prefer a rich loamy soil, so a sandy soil may not be holding enough moisture to endure the heat.
Kousa Dogwood also is not very drought tolerant. You will need to water your shrub unless you amend your soil.
Here is a link with more information.