Sounds like leaf spot disease a type of fungus encouraged by wet weather and poor air circulation. First collect and remove all the spotted leaves. Put a fresh mulch of bark chips under the plant to seal in any fungal spores that may be left on the ground. Rhododendrons set buds in the fall so water deeply at least once a week during the hot dry times of August and September. If you still have disappointing blooms next spring I would dig up that rhodie and move it or lose it. Life is too short to put up with unhappy plants. Keep growing, Marianne Binetti