You don't say what kind of climate you are in..

pruning after the buds form, which removes the buds for next year

an unusual cold snap before plants have hardened off in Fall, or a late Spring freeze can sometimes freeze buds and they don't open

lack of moisture during the late spring and summer reduces bud formation - mulch helps retain moisture and also keeps the soil cool

shade reduces the number of buds (have your trees grown to gradually shade them out)

a phosphorus deficiency, characterized by dull, dark green foliage with reddening underneath, reduces the number of buds, and poor plant nutrition in general reduces the number of buds

deer and/or squirrels ate the buds

There are two cases, the buds never formed or the buds did form and they never opened.

Failure to set flower buds may be a sign of too much health and vigor in a plant. One solution my be to prune the roots by cutting around the plant with a spade or moving the plant. This will check foliage growth and encourage production of flower buds. Application of nitrogen rich fertilizers are the main cause of vigor which suppresses flower bud production. Deadheading flowers as soon as they wilt can promote flower bud production. Too much shade, a cool wet summer, or inadequate phosphorus or potassium in the soil may also suppress flower bud production. There are a number of other reasons for a lack of flowers. The effect of each variable depends upon the variety of the plant. The effects include:

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.

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.

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.

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.

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.

Exposure. 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 more foliage and less flowers.

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.

There are many other cultural variables that influence the plant's health and hence, its ability to produce flowers.

Failure of flower buds to open could be due to a number of reasons. On a mature plant if they ever bloomed they will have a few of the seed pods still here and there. If you can't find any old seed pods, then they may have never bloomed. In any case, here are a few suggestions that may help:

Bud set. The buds could be foliage buds rather than flower buds. In this case check the previous section about flower buds not setting.

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.

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.

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.

Acidity. Measure the pH and acidify if necessary. Flowers of sulfur (powdered sulfur) is the best chemical 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.

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

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