There may be several reasons for decline but we cannot say for sure. Dogwoods grow best in morning sun and afternoon shade in a well drained soil. They are understory trees.
You may be dealing with drought, poor planting techniques, planting too deeply, poor drainage, root issues, etc. Branch dieback can be also attributed to cankers, anthracnose, powdery mildew, and insect issues.
Here is our dogwood publication for more information http://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/hgic/HGIC_Pubs/TreesandShrubs/...
Your dogwood does not look happy and you will have to consider replacement. If you decide to plant another dogwood, replace in the spring. See our website for the planting process and aftercare https://extension.umd.edu/hgic/plants/planting-process. Select dogwood varieties resistant to powdery mildew and spot anthracnose. See above publication. You may also want to select another type of tree suited for the location.