It could be any of a number of things. Is it planted under trees? If so, what kind? Azaleas flower poorly if planted under trees like maples, that have dense foliage.
Did you prune it last year? Pruning too late in the year will remove the flower buds.
Did you fertilize it? Too much nitrogen can prevent flowering.
If you had a period of warm weather this spring, followed by more cold weather, the flower buds may have been killed.