Experts and BHG readers answer.
non- flowering hydranga
I think you are referring to the large-flowered mophead hydrangeas Hydrangea macrophylla. I suspect that your hydrangeas are being damaged by cold weather. Most hydrangeas bloom on last year's wood and set their flower buds before the winter. If these are blasted by cold weather, they will not develop come spring. I also wonder if you are pruning the plants? Cutting down the old stems removes the flower buds at the same time, encouraging strong young growth from the base that will not bloom until a year later. You would be wise to select a cultivar such as 'Endless Summer' that produces blooms on both old and young growth. It would be better suited to your climatic conditions.