There are a number of possible causes, though a soil nutrient deficiency isn't as likely as other factors (drought stress, increasing shade from any nearby trees, a weakening due to pest or disease damage, winter bud death, etc.). Kousa dogwoods can be long-lived trees, but both native and Asian dogwoods can be stressed by hot, dry conditions, especially if they happen to be growing in full sun or surrounded by lawn that competes with their roots.

Last year, most of Maryland experienced a drought (even the year prior was somewhat dry), and it has not yet abated (see linked map). Was the tree irrigated occasionally? Spring-blooming trees develop their flower buds the summer/autumn prior, and those buds lie dormant on the branch tips until they open in spring. Stress experienced during that bud development period can cause the plant to forego forming buds, or already-developed buds might abort during winter if they dry out too much. Therefore, the overall condition of a tree months prior to its flowering period can impact how well it blooms.

Did the canopy have fewer or stunted leaves in addition to not flowering much? If so, that suggests that there may be a larger health issue beyond transient stress that impacts flowering. Is the tree surrounded by lawn? If so, do mowers or string trimmers ever hit the bark and create wounds? In that case, there may be a wood-boring insect or canker infection that took advantage of the wound and is causing dieback or decline. Such conditions tend not to be treatable, but if you don't see damage and the leaves emerge normally, then it's probably not a factor.

Are there any mature trees nearby which have grown and cast more shade on the dogwood? Dogwoods thrive in a setting of mixed sun and shade, but more shade can reduce flowering, which is normal. Trees might then only flower well every other year or so, although the situation with your tree sounds more like a consistent, gradual decline in bloom rather than a fluctuation. Here too, that would suggest a health issue is the cause rather than a fairly harmless change in sun exposure.

Although images of the tree might not help much with diagnosis right now (though you're welcome to send them), once it leafs-out, feel free to share pictures of any concerning symptoms. The situation might require a visit from a certified arborist or licensed tree expert to help with a diagnosis; they can also look for sources of tree stress that might be able to be alleviated. Arborists can't cure every issue or necessarily reverse tree decline, but they may find symptoms that will be difficult to see in photos.

If other causes of decline are ruled-out, then fertilization might help. (Plants suffering from a pest or disease problem should not be fertilized, as that can worsen the problem.) However, what fertilizer formulation to apply will depend on the results of a laboratory soil test, which measures soil nutrient levels as well as checking acidity level (pH). Dogwoods prefer acidic soil, and if the pH in its root zone has risen over time (such as if a nearby lawn is limed regularly), then their roots will have a hard time absorbing the nutrients they need, regardless of how plentiful those nutrients may be in the soil. In that situation, the solution would be to lower the pH with soil sulfur applications rather than applying fertilizer.

Miri