Answer: Based on your description I suspect they may have yellowed due to planting too deeply and/or overwatering and lack of direct sun combined, as well as possibly a fertility problem -- either over or under supply of nitrogen or possibly a micronutrient deficiency. Blooming last summer would have been dependent on buds formed on growth from the year before, so a one year performance is not really a fair trial. On the other hand, they do need some direct sun to bloom well. A few hours of morning sun up to all morning sun is about right. I would consider moving them and I would not re-plant with hydrangeas there.
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