Answer: When plants fail to thrive it's almost always due to growing conditions, either soil or exposure to sunshine. Even though the conditions appear to be the same for all your hydrangeas, there's obviously something different or your plants would have done something besides just sit there over the past 3-4 years. I'd dig the affected plants and either plant them in containers with fresh potting soil or spread some organic matter over the planting bed and dig it into a depth of 6-8" then replant the hydrangeas. Sometimes just the digging and replacement of the plants will wake them up and start them growing. But, amending the soil so it is nutrient rich and easy for the roots to penetrate will help as well. Water them deeply once a week after the transplant. I'm sure you'll see new growth later in the season.
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