Clethera bush leaves dying
This looks to us like poor establishment and/or a root issue. It does not look like a disease or pest problem. When you installed the plant two years ago, where they root bound in their pots? Did you loosen up the root ball and set the roots outward in the planting hole? Some plants start out tightly bound in their nursery pots and fail to establish into the surrounding soil. They will survive for a year or so but then begin to fail if they aren't' getting sufficient water/nutrients due to poor root establishment. You could pull up one of the symptomatic plants and check the root system. If it pulls up still in the shape of the original container, that is a problem. The roots should look white and fleshy rather than dark brown/black.
Clethra likes moist soil, rich in organic matter. It grows naturally in wet locations. Lack of sufficient moisture, especially during the establishment period, may have also contributed to the problem.