Answer: It sounds like a nutritional problem. I assume the plant is in the shade for most of the day (as it would like to be). I would suggest you start with a soil test. Your County Extension office (ph# 334-937-7176) can give you instructions on how to submit a soil sample for testing. High pH, soil drainage problems and root rotting fungi can also cause nutrient deficiency symptoms in the plant.
You didn't mention the species or variety of hydrangea. This could have some effect on size. Nevertheless, I still think the problem will surface in your soil test.
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