Are you ready: You may be too good to these plants! One source suggests these causes:
Too much shade. This is very common in North America where, in order to regulate sun and soil temperature, plants are placed in deep shade. This allows healthy, if straggly growth, but can inhibit flowering. The more light you can give a plant, the more likely it is to flower, so there is a trade-off between the need for shade and the need for light.
The variety takes many years to flower (it does not sound like this is your situation).
Kindness. Rhododendrons flower in order to reproduce. A contented, well-fed, well-watered, well-shaded plant may not feel any need to reproduce, as it perceives no threat to its survival. Do not feed after mid-summer, as this encourages growth at the expense of flowers. Nurserymen cut down watering in late summer to stress plants into flowering the following year.
Are you ready: You may be too good to these plants! One source suggests these causes: