Boxwoods are unfortunately vulnerable to several pests and diseases (though few plants are pest-free, to be fair); the linked page presents these symptoms and discusses management options (when treatments are practical). Proper pruning is key to help maintain boxwood health. It looks like these have been sheared for many years. The video attached, will show you how to properly prune to open up the plants and increase airflow. 

A fungal Volutella infection can be quite common as well, as is dieback from root decline (such as from overwatering or recurring drought stress) or vole damage around the base of the main stems (especially if heavily mulched). 

There are disease-resistant boxwood cultivars on the market these days (though they are in the minority), but none are immune to these or other ailments common to boxwoods. Worst-case, if the management of recurring outbreaks of these pests or a disease is too tedious or expensive, consider replacing the shrubs with a different species. Better yet, hedges or other mass plantings or plant groupings are best planted with a variety of species instead of a single species, which makes the planting more resilient in the face of potential problems since not all of the species in the planting will have equal vulnerability to the same issue. Always try to select a species that fits the site space availability well since, even though some shrubs are quite tolerant of pruning, they will be healthier and lower-maintenance overall if allowed to mature to their natural size (or close to it). Fortunately, many shrubs have dwarf cultivars available since small-maturing plants are currently popular.

If you can send us a close up photo of the leaves we may be able to see specific issues that need to be treated (if treatment is warranted). Let us know if you have any further questions with the care for your boxwood.

Emily