It sounds as if you have had this rhododendron for more than two years, Millie, and that it used to flower well.
It is quite a large plant and I suspect that it may have outgrown its container and compost. At the end of the summer I would suggest that you take it out of its container and make sure that it is not pot-bound. Then repot it in a suitable sized container with fresh, ericaceous compost.

Hi Millie, are you feeding it? In a container a rhododendron needs an ericaceous feed. If your rhododendron is in an area that gets a lot of frost then the flower buds could be getting frosted. This happens to some of our rhododendrons every year.

My gardening friend here in Scotland said lots of his customers have found the flower heads rotting off Rhododendrons due to the cold and rain early in the year.

Thanks for all suggestions will try repotting and new ericacious compost as do feed regularly when required have only had this plant two years bought it from G.W magazine offer.