This can be either of two problems. It is producing flower buds, but they aren't living through to spring, or it isn't producing flower buds.

In the first case, where the flower buds aren't viable it could be:

1) Bud blast. Plants which are not sufficiently hardened off or are exposed to unseasonable warm spells can start bloom prematurely. These blooms are seldom satisfactory and many times get frozen before opening fully. In any case, the seasonal bloom is lost. Also, disease may attack the buds before they open.

2) Low temperatures. The buds could be flower buds that froze during the winter. Cold climates are too cold for many rhododendrons. Most rhododendrons have a low temperature at which the flower buds are damaged and will not produce flowers. It varies greatly from variety to variety. Flower buds can also be damaged by cold, dry winds, particularly when warm winter weather is followed by a period of bitter cold.

3) Nutrients. Improper nutrients my be a problem that affects cold hardiness and flower bud set. A few things you can do are to fertilize with phosphorus (super-phosphate) per directions to increase hardiness and flower bud set. This can be done any time. Do not use nitrogen rich fertilizers as they may inhibit flower bud production and also reduce cold hardiness. Lawn fertilizers are notoriously high in nitrogen and should be kept away from flowering plants.

4) Acidity. Measure the pH and acidify if necessary. Flowers of sulfur (powdered sulfur) or iron sulfate are the best chemicals to use to increase the acidity [lower the pH]. Do not use aluminum sulfate since aluminum salts build up in the soil and eventually becomes toxic to many plants including rhododendrons and azaleas. If soil is too acid, the symptoms can be the same. Very acidic soil can prevent the roots from taking up nutrients. As many of my rhododendrons are planted in very acidic forest soil, an application of Dolomite and a light topdressing of mushroom manure in late spring is all they need. Sprinkle the lime on in late winter, very early spring. Don't overdo it - just a light sprinkle. If it is mid-spring, get the lime on right away so the rhododendron roots will be able to take up the soil nutrients in time for new growth. If you don't have rain, water it in well.

5) Protection. If the plants are wrapped in burlap during the winter, they may gain a few more degrees in hardiness.

6) Drought. When soil moisture is too low, the buds will not open. Watering will usually resolve this condition if detected soon enough.

7) Deer Damage. Deer and rabbits may eat many of the flower buds as they browse in the winter, particularly if the weather is harsh and other food is scarce.

In the other case when no flower buds are produced.

1) Pruning. The buds are formed in late summer and early fall so pruning then or later is not advisable since it will remove flower buds. New leaf buds will form in the spring, but new flower buds won't form until the next year.

2) Variety. Some plants will never bloom. Some rhododendrons that come from the seed of a hybrid plant will look good but will never produce flowers or will produce very poor flowers. To come true to the parent plant, a hybrid may be propagated by cuttings or tissue culture but not from seed. A good hybrid seedling only comes about once in a while. For that reason it is important to know that you are getting a good named variety or a good species.

3) Fertilizing. Nitrogen promotes leaf and branch growth and discourages flower bud production. It can also force late season growth that gets killed or stunted by frost damage. Phosphorus promotes flower bud production and hardiness. Potassium is necessary for well being.

4) Weather. Cold weather can kill flower buds. Usually you see the brown buds in the spring. Cold spells in the fall or spring can damage buds that are not hardened off. Bud blast (blooming in fall or winter) uses up good buds which are then not available at the normal blooming time.

5) Age. Most rhododendrons take 2 to 3 years to bloom from a rooted cutting unless forced. Some take longer and some bloom sooner. From seeds the plant may take 1 or 2 additional years.

6) Sun & Shade. Some rhododendrons need full sun to bloom and others can take fairly dense shade. In general, the more sun the more flower buds but also the greater exposure to damage from desiccation in summer or winter. More shade produces tall spindly foliage and less flowers.

7) Inspection. You can usually tell if the plant has ever bloomed. A rhododendron that has bloomed will have the seed pods on it unless it has been dead-headed. If dead-headed too late after blooming, new flower buds can be damaged.

For this and more visit the link below:

Here is a link that might be useful: How to grow rhododendrons.