Most likely reason in Zone 5 is damage in winter or early spring killing the flower buds. Also, pruning at the wrong time of year. Not enough sun could be a possibility.

This happens to me whenever we have late frosts and you don't cover the plants. They are very touchy when it comes to frost. Therefore, everything will look great on the plant but no blooms.

This has happened to me twice and whenever I hear of possible frost the hydrangeas get covered.

Three of mine are blooming but one isn't. I think that one wasn't covered well enough.