It's hard to identify the plants at that distance, but the evergreens appear to be Arborvitae, which require a full sun exposure to thrive. (Full sun being a minimum of 6-8 hours of direct sun in summer.) There are not many evergreen shrubs or small trees that thrive in understory conditions where taller trees shade them for part or most of the day.

Pine needles make a great free mulch and they are not a cause of any plant problems, and they do not acidify the soil to any meaningful degree. (If anything, that would be a good thing if they did, because many plants prefer slightly acidic soil.)

Root competition with the mature trees may be part of the problem, aside from insufficient sun. For the past couple of years, most of Maryland has been in a mild to moderately severe drought, and unless the plantings have been watered periodically (more often if the soil is fairly sandy-textured or low in organic matter), drought stress may be adding to their struggles. You can learn more on the linked pages.

It's hard to say what to try growing since we don't know what "not great soil" means (does it not drain well and stay wet? is it just sandy and lacking in organic matter?), although you could have a laboratory soil test taken if you wanted to measure nutrient levels, acidity (pH), and organic matter content. Do deer browse in the yard regularly? If so, you'll need deer-resistant plants, which narrows-down the options. What are the desired height and width (or at least limits on how tall the largest plants in that area can grow without having to be trimmed)?

Here are some starter ideas based on the semi-shaded lighting and what we presume to be average soil drainage:

• Anise Tree (it's really a shrub, but that's its common name; Illicium) -- there are several species and hybrids on the market, though since many gardeners in Maryland are not familiar with them, they may be hard to source; Illicium floridanum and Illicium parviflorum are both U.S. natives, though their native range is limited to the southeast; still, they are often winter-hardy enough to be grown in central and southern MD with few problems; they're evergreen and prefer shade year-round, so it helps that at least some of the trees above them are evergreen
• American Holly (Ilex opaca) -- native and evergreen; this would be a longer-term option since, despite their eventual large stature, these are fairly slow-growing trees; there are dwarf shrub varieties like 'Maryland Dwarf', but they are actually too low and too slow to screen much of anything (although could be a useful accent if desired)
• Mountain-Laurel (Kalmia latifolia) -- native and evergreen; slow-growing and may eventually lose its lowermost leaves as it ages, though the twisting branches that come with age can be showy in their own right; dwarf cultivars exist that stay leafier as they mature, but their final size is usually too short to block any views
• Florida Leucothoe (Agarista populifolia) -- evergreen; despite its common name (it is native in the southeast), this species is cold-hardy enough to do well in central and southern MD; mature specimens of this shrub have been growing at the U.S. National Arboretum in D.C. (a similar winter hardiness zone to your area) for years; like the Illicium, it's not well-known to area gardeners so may be hard to source; other species of Leucothoe are more readily available at garden centers, but they stay fairly short, but still may be welcome accents to a mixed planting
• Bayberry / Waxmyrtle (Myrica or Morella pensylvanica and M. cerifera) -- native and semi-evergreen, which means that they might retain most leaves or might shed most leaves any given winter (it's partially genetics and partly weather-dependent); while full sun is ideal for the fullest density of foliage, these shrubs do tolerate part shade well; mature height really varies, with nutrient-poor soil keeping them more compact; some cultivars exist but can be harder to source than the straight species
• Sweetbay Magnolia (Magnolia virginiana) -- native and either semi-evergreen or deciduous (like with the Bayberry, it varies); matures into a small to almost medium-sized tree and has a moderate growth rate (maybe 1 foot per year, perhaps more); usually multi-stemmed, but could have one trunk depending on how it was grown by the nursery
• Summersweet (Clethra alnifolia) -- deciduous; most compact varieties on the market might be too short if you want screening, but they can either be accent plants or you can use the straight species, which gets fairly tall compared to the varieties; very fragrant summer flowers and a potentially suckering habit, which means it might spread (though not rampantly) from new stems arising from the root system

Miri