Where are you generally and what is its garden situation in terms of sun and soil? How long has the plant been in this spot? What does the soil feel like a few inches down - dry, soggy, or moist but not super wet?

Western Pa zone 6, it gets about 6 hours sun. Soil is pretty soggy. Been in this spot 2 years

IME panicled hydrangeas like this aren’t happy with soggy soil. Is there another spot you can move it to early fall or can you adjust its current situation by helping water to drain from this spot better? Alternatively, this fall dig it up, add soil to raise the bed above that around it some, and replant. This will help water drain away from the spot and give the plant better drainage. Too much and too little water look somewhat similar.

Six hours should be enough sun for good bloom. Most likely being in the second year in your garden explains the single bloom, though water stress isn’t helping I am sure. It will usually take 3-5 years for a shrub to settle in well enough to get really good bloom since it needs to grow enough roots to support good bloom. If it is planted in the lawn, when you replant, remove grass over its roots and give it a ring of mulcha few feet across, though don’t place it against the trunk.