Dear Mr. and Mrs. Smith
Sorry to hear you are having problems with your dogwoods. Thanks for sending the photos, very helpful! It looks like some of the major limbs have died. Three major possibilities come to mind to explain this type of dieback:
1. Dogwood borer. If so, you might see sloughing bark and small holes in the trunk and branches (for more information, see http://www.clemson.edu/ extension/hgic/pests/plant_ pests/trees/hgic2003.html).