Experts and BHG readers answer.
No blooms on hydrangeas
"You don't say what hardiness Zone you're in or what species hydrangea. Some species of hydrangea will survive in cold climates but their flowerbuds are susceptible to late frosts. This could be the situation you are experiencing. Shade is also a culprit, but you say yours get plenty of sunlight, so that shouldn't be an issue here.
Luke Miller, editor
Garden Ideas & Outdoor Living magazine"