It is also important to determine if the shrub is not developing flower buds at all or if it developing buds but these do not open. Each of these two conditions has unique possible causes.

I would make sure the plant gets several hours of sun, check for pests (squirrels, deer, etc who like to eat flower buds), check the soil pH and check for phosphorus defficiency.

If you fertilize, check if your fertilizer is nitrogen rich. Azaleas need little fertilizer normally and a nitrigen rich fertilizer will tend to keep the plant growing nice green leaves at the expense of flower buds.