Answer: These hydrangeas are special in that they bloom on both old and new wood. Old wood means stems that grew the year before. Flowers would form early in the season on old wood, and then later on in the summer flowers would form on new growth of the year.
Based on your description of new growth being only from the base, I suspect winter damage has killed back the old wood and thus prevented them from flowering on the old wood. Sometimes too, spring frosts will damage the new growth as it emerges in the spring. Pruning at the wrong time (fall, winter, spring) will also remove the old wood, thus limiting early season flowering. The only spring pruning would be to remove winter-killed branches, being very cautious and patient and waiting until early summer before deciding they are truly dead.
Even if the old wood is lost to the cold, there should be blooms on the new growth. If your plant is not setting buds, it may be that it needs more sun. This is often true in northern areas with short growing seasons. You might consider moving it to a location with full sun all morning and through noontime or for even longer. If you do this, make sure the soil is evenly moist yet well drained, meaning damp like a wrung out sponge. If you need to move it, I would suggest doing so in very early spring rather than now.
In your climate, you might be able to protect the plant during the winter to maximize the bloom performance. Make sure it is planted in a sheltered location with protection from winter winds. In late fall after it has gone dormant, mulch over the root area and also takes steps to insulate the branches. To do this, some gardeners put a burlap wrap around the perimeter of the plant and stuff it with nonpacking material such as oak leaves or straw, or you could use a wire mesh cylinder and stuff that lightly with leaves or straw. Unfortunately, in your cold climate, even this may not be sufficient. Your zip code places you in zone 5A, the coldest part of zone 5. Depending on your microclimate, it might even be as cold as zone 4, so the insulation may not be quite enough to bring the old wood through. Your local professional nursery staff should be able to tell you if it is worth the effort trying to do this based on their knowledge of your local weather and this plant's performance.
Your plant may still bloom yet this summer/fall. I hope this helps you trouble shoot.
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