The correct answer depends on the type of hydrangea that you are talking about. Hydrangea macrophylla, also known as Big leaf hydrangea, French Hydrangea or Hortensia may have blooming issues due to some of these problems:
* macrophyllas develop invisible flower buds anywhere from mid summer to early fall 2020 (varies depending on one's geographical location) and then the flower buds open in spring/summer 2021. However, if you were to prune after the buds have developed then you could cut off this year's flower.
* Winter is zone 6: If winter is too cold, the drying cold and temperatures may kill the stems. Macrophyllas that bloom only on old wood need to be winter protected so the stems do not die.
A rebloomer macrophylla may be more bloom reliable. These varieties will produce flower buds the same way but it will also produce a second flush of summer 2021 blooms from new growth (stems) that started growing in spring 2021. They need to get sufficient sun to let those stems grow tall and old enough before the end of the growing season so I prefer using rebloomer varieties that do not grow very tall like the Let's Dance Series of reblooming macrophyllas.
Oakleaf, paniculata and Annabelle-like hydrangeas tend to be more winter hardy and bloom reliable.
* Pests like deer, squirrels and bunnies can eat the flower buds in spring 2021.
* dense shade can reduce blooms.
* too much nitrogen rich fertilizer can promote dark, green foliage at the expense of blooms.