There are a number of possibilities:

First, I hope you didn't prune it back. The flower buds are formed in early summer, so any pruning after early June can remove the flower buds for the following year.

Second, fertilizer usually promotes plant health at the expense of flowers. So I wouldn't fertilize and see if you get flowers 2 years from now. The buds for next years flowers will form in early summer, so if you fertilized this spring, you may have the same problem next spring.

Third, the rhododendron may not be getting enough sun in early summer to form flower buds. Some varieties don't need much sun, but others are very shy about blooming in too much shade.

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

Here's the key question: are there flower buds present in the spring which fail to open or is the plant not setting flower buds at all? If it's the second - no flower buds set - then rhodyman has covered the possibilities. If there are flower buds which never open, then there are other things to consider: Marginal hardiness which causes the buds to be winter killed. Late frosts after the buds have begun to swell which kills them. In these cases, promising buds remain small and turn brown. Flower buds eaten by deer or squirrels. In this case, plump buds present in the fall diappear over the winter.