There are two main reasons why Hydrangeas fail to bloom. Hydrangeas produce flowers on old wood in the fall.. Therefore, if they are pruned at the wrong time of year you may actually be cutting off next years flowers.
The second biggest cause is damage from winter cold. If the flower buds are damaged in the winter, they may not be able to bloom in spring.
The best time to prune Hydrangeas is immediately after they have flowers (usually mid-summer). However, you can remove dead, damaged or diseased branches at any time.
Another option is to consider the Endless Summer varieties of Hydrangea. These new types produce flowers on old and new wood therefore are much less susceptible to poorly timed pruning efforts.