This could be caused by a number of reasons.
Too much Sun? Hydrangeas like Morning sun and afternoon shade.
Improper timing on Pruning? Many Flowering Hydrangeas form their flowers on old growth. If you prune this old growth off before it has flowered, you will more than likely have reduced your number of blooms for the season.
Water? The first part of Hydrangea is Hydra…meaning Water! These plants need to be supplemented with moisture due to the fact that they have such large blossoms.
Fertilizer? Flowering plants need to be fed in order to get blossoms.