I have 5 variegated red-twig dogwoods on a south-facing burm in our front yard in SW Michigan; all are doing well. However, in one of the bushes there are several large branches which have leaves with essentially no color. I trimmed one out one similarly affected branch last fall but this year, in addition to one peripheral branch there are 6-8 centrally located branches with the same appearance.
Sounds like your dogwood has some reversion. Variegated leaves are caused by a mutation that makes part of the leaf contain no chlorophyll (the green pigment in leaves and stems that enables photosynthesis). Occasionally a branch or two of a variegated plant will revert back to the original all green color. If these branches are not pruned out promptly, they can overtake the rest of the shrub. They are more vigorous because of the greater amount of chlorophyll and better ability to photosynthesize.