Here are some ideas why your hydrangea is not blooming:
1. Pruning too late
Hydrangea
flower buds for next year will begin to develop between August and October. So if you prune in November or early
spring, you would be cutting off next year's blooms. Since it is hard to tell if new buds are developing, it is recommended that any pruning be done a few weeks after blooming starts on varieties that bloom on old wood.
If you have a variety that blooms on new & old wood, like Endless Summer, this should not be an issue.
2. Weather issues
Hydrangeas in northern areas require some type of winter protection in order for the
flower buds and/or stems not to freeze. But the danger is not limited to
zone 4/5ers only though.
You could be hit with an early or late freeze. For example, hydrangeas in areas like northern Tennessee will suffer from the tendency of the temperatures to fluctuate wildy in the Fall.
Sudden drops here in Texas could cause damage to the buds if the temperature drops below the
plant's temperature requirements. In December of 2005, the temperatures fell down to around 10 degrees or lower in the Dallas/Fort Worth Metroplex. Because we were also in the middle of a drought, many hydrangeas were weak and the cold weather affected their buds. Crape Myrtles also suffered.
3. Florist hydrangeas, sold thru grocery stores and florists, are weak cultivars meant to be enjoyed inside the house. They are bred to grow in nurseries and to bloom early. As a result, they can easily decline if planted outside. Florist hydrangeas are the ones sold in pots with colorful aluminum foil around Mother's Day and other holidays.
4. Old MACs need pruning every five years or so
After five years or so, the bloomage of some hydrangea Macrophyllas will decline. To rejuvenate the
plant, you need to cut 1/3 of the oldest stems to the ground on year one, cut the next third on year two and cut the rest on year three.
5. Go easy on nitrogen rich fertilizers.
Make sure the lawn fertilizer is not falling in the area where the hydrangea is planted. And watch out for chemical fertilizers like Miracle Grow for Azaleas/Camellias/Rhododendron which is 30% nitrogen and whose labels says to re-apply it every 7-14 days.
Hydrangeas are not hungry
plants like
roses. An application of composted manure or cottonseed meal in April and another in late June is all they need in your area (northern climates should use only one application in May).
6. Take good care of them when they are creating buds.
For hydrangeas that
flower on old wood, this means August thru October. It is important that the soil be moist (wet soil promotes fungal problems in the leaves and/or roots), dry soil can kill the buds and/or the
plant).
Wilting during the morning hours indicates that the
plant is not getting enough water. Wilting in the afternoon during the summer is ok provided the
plant recovers by the next morning. Lack of water can greatly reduce
flower bud production.
And do not forget to water during the winter months albeit at reduced levels. Our ground does not freeze so their roots stay active during that time. Lack of water during that time could kill your buds.
7. Not enough sun
Some hydrangeas will not bloom if you
plant them in full shade. To corect this, you may need to transplant them or figure out a way for them to get a few hours of morning sun or dapped sun.
8. Planted in the wrong
zoneNamed hydrangea cultivars should have a label that says in what
zones they can be planted. For example: if the label says it is good thru
zone 7 and you
plant in
zone 6, the
plant may prosper but produce no blooms. However, I do not think this applies to you.
Regarding snapdragon's question, iron pellets or products containing iron chelates are only used to correct a condition called iron chlorosis. When you have iron chlorosis, the leaves turn yellow but the leaf's veins stay green. Lowering the soil Ph by adding iron or lowering the soil Ph corrects this problem. Personally, I add liquid products containing iron chelates. Say, something like Green Light Iron And Soil Acidifier.
Regarding the question about color change, be aware that it is much easier to control the color of a hydrangea in a container than in the ground. If you do not evenly apply the chemicals around a
plant (or around several
plants) then some roots may not absorb as much minerals as others. You basically end up with different levels of acidity and different amounts of minerals in the soil around the
plant(s). And yes, that means blooms with different types of blue (or pink/red).
Color intensity is a function of waaay too many factors that we consumers cannot control very well when planting in the ground; things such as the
plant variety,
plant health, the weather and even the amount of minerals already in the soil.
For example: to get blue in my alkaline soil, I would have to get frequent soil tests (to see how much I have of certain minerals), I would have to acidify my soil & add aluminum if my soil has too little (most people do both by adding aluminum sulfate or you can only acidify by adding sulphur), and I would have to use a low phosphorus/high potassium fertilizer! Easier said than done, hu!
Wholesalers in their nurseries do better than we do but then again, they
plant in pots! For us, it is a process that takes years and, unless we are next door neighbors, the process and mineral amounts needed would be different.
Once you get it right, you have to maintain the acidity and mineral levels at the current level year after year by re-applying the stuff in similar amounts again.
I hope that helps,
Luis
Why? They are in a spot with mostly shade but partial sun. They are planted 2 feet from the house in a mostly clay-like soil that has had organic matter added every season. Anyone know what the main needs hydrangea have in order to bloom? Is it too little phosphorus? I have had three of the 
:)Sue
The #$%* DEER ATE ALL THE TIPS ON IT the other night! 
