There are several reasons why they may not be blooming. Late frosts, over pruning, not enough sun and fertlizing are common causes. Over fertilizing seems to be the most common reason. If your hydrangea looks green and healthy, you could be fertilizing too much or with the wrong fertilizer! High nitrogen fertilizers promote lush, leafy growth but can inhibit blooming. As a general rule, fertilize once in early spring and once in early fall. Hydrangea will bloom best if they are starved a little bit. Many people do forget that blue hydrangeas require soil acidity more then they require high amounts of fertilizer. Acidity can be added to the soil with products like Bonide’s Aluminum Sulfate, and Espoma’s Soil Acidifier.