Hydrangeas are famous for prolific flower clusters, so it is particularly disappointing if they don't bloom. Under the proper conditions and with appropriate care, this can be prevented.
A common reason for no blooms is improper pruning. Many varieties should not be pruned at all. Some varieties bud on old wood, some on new. The buds on many varieties form in August and September for the following summer's blooms; if old wood hydrangeas are pruned, flower buds are destroyed.
Many hydrangeas are cold tolerant, but some varieties need winter protection. A late spring frost can destroy the buds on even tolerant hydrangeas if the hydrangea has come out of dormancy when the frost hits.
Not every variety of hydrangea is appropriate for all regions. Matching varieties with their suitable zone can be done on websites like All About Hydrangeas.
Plant food that is high in nitrogen will prevent flowering. Use a balanced fertiliser.
If the plant has been weakened from disease or a pest infestation, the buds might be dead before they are even noticed.