Some of the boxwoods have a lot of brown stems in the middle of the mature plants. Are these plants a dying or can they survive and fill in green again?
Certified GKH Gardening Expert
This could be caused by environmental issues, pests or disease.
You will need to inspect the plants more closely.
It's a good idea to remove and dead material and clean up the area around the plants and dispose of the plant material.
Here are some articles that will help.