Very strange. I would normally think that they are a different variety or perhaps in a different amount of light. If not then I suggest you fertilize all your plants with a product for acid loving plants such as Miracid or an Azalea/Camellia food following label instructions. Water the fertilizer in well. Consider brightening the shade some by removing a few limbs on the trees above them. Nutrition and sunlight will go a long way toward encouraging them to be more vigorous and set more blooms. Don't prune the plants after early summer.
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