It's hard to tell what caused the leaf sparseness and branch tip dieback in some places, though it's likely a gradual decline from overlapping factors. Rhododendrons and their azalea cousins are susceptible to several ailments, notably Botryosphaeria canker when they are stressed (such as from drought). Are the shrubs monitored for watering needs during dry spells? Last year, for example, much of Maryland experienced prolonged drought for most of the entire growing season. Is the shrub near a roof downspout outlet or leaky hose faucet? Soil that stays too wet can also cause root loss and branch dieback, so sometimes it's hard to determine which condition may be predisposing the shrub to decline.

You can use the information on our rhododendron diagnostic page to look for similar symptoms and likely culprits. Sometimes certain issues are secondary, like stem-boring insect larvae, since they target plants already under stress (even if that stress isn't obvious at the time).

All you can do for now is to prune off any dead branches that have no foliage at their tips. We can't see the planting site, so it's hard to recommend what else might be helpful to try, but if they are getting heavy shade in summer, that could be exacerbating leaf loss on the older wood. There is no fungicide which would be useful at this point, nor would an insecticide or fertilizer provide much benefit. While harshly pruning an older rhododendron back to force regrowth sometimes results in a renewed appearance and vigor, it doesn't always work and it's hard to predict how healthy the roots may be to attempt it. (In that case, several years of growth would be needed to regain some size and flowering potential.)

Miri