Why isn't my hydrangea blooming?

In the Midwest, the issue of hydrangeas not blooming typically concerns bigleaf hydrangeas (Hydrangea macrophylla). There are usually one of two main reasons why shrubs might not bloom profusely: 

1. The shrubs were pruned at the wrong time: Bigleaf hydrangeas bloom on "old wood", meaning that their flower buds were set the previous year. Shrubs pruned between late summer and spring of the following year will remove flower buds, contributing to little to now flowering. Shrubs that need pruning should be pruned after flowering, approximately June to early August. 
    
2. Flower buds were damaged by winter temperatures: Bigleaf hydrangeas are hardy in the Midwest, but flowering abundance can vary greatly from year to year. Since their flower buds are produced on the previous year's growth, the buds are often damaged by severe winter temperatures or fluctuating temperatures in early spring. This leads to sparse blooming. If blooming continues to be inconsistent, consider replanting the shrub with more reliable blooming hydrangeas, including oakleaf (H. quercifolia), smooth (H. arborescens) or panicle (H. paniculata) hydrangea species.