The azaleas appear to need rejuvenation pruning to restore a denser appearance and to encourage more leafy growth. Are any receiving lots of summer sun? (You mention the pine shade over the white-flowered azaleas, but we can't tell how much direct sun the pink plants by the house are getting. Western exposures are usually pretty sunny if no mature tree is nearby.) If so, azaleas become stressed and less tolerant of drought (not that they are highly drought-tolerant anyway) when grown in too much direct sun, and one reaction they have is to develop leaves over time that are smaller, which gives the plant an overall twiggy, spartan look. The best long-term solution in that situation is to move plants into a bit more shade, like the dappled light of a tree.

Are the plants monitored for watering needs during dry summer and autumn periods? Last year, for example, most of Maryland experienced significant drought, and that can impact plants for months and years after weather patterns become more normal, unless they were being irrigated. Are any of the azaleas near a roof downspout outlet or source of surplus water? Like drought stress, over-watering and soil that doesn't drain well can kill azalea roots, leading to branch dieback and sparse foliage. Even though they've been in their current locations for a long time, gradual changes in soil conditions or drainage patterns are possible, and sometimes cumulative stresses push a plant over its tolerance limits when it didn't seem to be bothered by something in years prior.

If the plants are pruned regularly to maintain a certain size or shape, that can, over time, cause the lower portions of the shrub to lose leaves (and not regrow them) because the upper branches are blocking too much light. While not a health problem for an azalea, this can also contribute to a leggy, sparse look. Rejuvenation pruning can help correct this.

Black sooty residue on bark may be sooty mold, a fungus that does not infect plants but which thrives on sugary residues from sap-feeding insects like scale, aphids, and lace bug. The latter insect is a fairly common pest of azaleas which are stressed by sun or drought. We can't tell what the cause of the dark bark is from the photos, but you can inspect plants for bark scale based on information in the linked page above. If you decide to do a rejuvenation pruning, that would remove the vast majority of any scale present, so treatment might not be necessary even if they are present.

Rejuvenation pruning might look alarmingly drastic, but it removes most of the old wood and forces new growth to take its place. (An example of a boxwood pruned this way is attached.) Either now or in late spring, once blooms are finished, you can cut all the branches down to roughly a foot above the ground. Use sharp pruners or loppers so the branches are not cracked and split in the process; if any stems are thicker than loppers can handle, try a folding pruning saw, since they can get into tight spaces more easily. Azaleas are not particularly fast growers, so it might take a shrub a few years to look full and more normal again, and blooming also may be interrupted by a year or two.

A compromise may be to thin the plants instead, which entails pruning some of the oldest wood back about as far as with rejuvenation pruning, but the difference is that only about a third of the stems are cut back any given year, allowing some of the older growth to remain for the enjoyment of flowers or plant size. After three years, you'll have removed the last of the taller old wood and the regrowth has had a bit of time to start filling in. The trade-off is that the plant may look a bit awkward mid-process compared to just taking all stems back at one time.

Miri