do you live in? If you are in a zone
that can get late freezes like me (I'm in zone
6/7) that may be the problem. Hydrangeas bloom on old wood, which means they bloom on last years growth. Late freezes can kill those buds and you won't get flowers
. I've slowly been replacing all mine with the Endless Summer hydrangeas which blooms on old and new wood, so even if we get a late freeze, they will still bloom on the new growth. As far as pruning goes, only prune right after they flower
or you could be cutting off the buds for next year's flowers
. I wait until mine have fully leafed out in late spring
and just prune out the dead wood.