More info might help people answer.
What kind of Hydrangea is it?
When do you prune it?
Where is it growing, ie soil type, aspect etc?

If it is a mophead hydrangeas (one of those with round flower heads rather than cone shaped ones) it could be you are pruning it wrongly and cutting off the shoots that would have flowered. Try not pruning it at all this year and see if you get flowers next year. If you do the answer is to leave the dead flower heads on over the winter and then to cut off only those, leaving plenty of leaf buds on the stem - try at first just cutting off the flower heads themselves with perhaps about three or four inches of stem. If you need to control the size of the plant for any reason take out several longer stems each year rather than shortening them all at the same time.

If its in a pot and been in it for several years you could also try repotting it with new compost.

steragram has given you good advice Ivy, I always leave the flowers on all Winter, then you know which bits to cut off.The other thing is this, if they are in pots like mine, are you feeding them so they make loads of great foliage at the expense of the flowers?I made that mistake once.