Some hydrangeas bloom on new growth and others bloom on last season's growth. If the hydrangeas which bloom on last season's growth die back because of a particularly cold winter, then they will not bloom the next year. I have two of those hydrangeas and neither is blooming this year after this past winter's very cold temperatures. So depending upon your particular type of hydrangeas, winter kill of last season's growth could be the culprit.
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.