How can I encourage are hydrangeas to blossom?

Hydrangeas do not like nitrogen so much. Never mulch them with grass clippings for example. They need calcium and potassium to encourage blooming. I put my egg shells and banana peelings with coffee grounds in the blender with a little water and pour it around the base of the bush. Also, when they do bloom, be sure to prune them back as soon as the blooms are done. It helps them. It is a good time now in the fall to start the "blender " feeding to help it get ready for spring/summer. Hope this helps

They need phosphorus to bloom...have you soil tested the area? Not enough sun?

https://thistlewoodfarms.com/how-to-make-your-hydrangeas-bloom/

https://www.gardenguides.com/84033-make-hydrangeas-bloom.html

http://www.millionplants.com/advice/wont-hydrangea-bloom/

https://www.thisoldhouse.com/ideas/5-tips-growing-gorgeous-hydrangeas