A common cause of a hydrangea not blooming is because it is being over fertilized. Believe it or not, too much fertilizer is not a good thing. If you over fertilize the plant, it is going to encourage more leaf growth, but not flower production. Fertilize once in the spring, and once in the summer with a hydrangea fertilizer, then leave it alone. Make sure it is getting enough water, and not too much sun, and you should get some gorgeous blooms.