Answer: It's difficult to find plants willing to tolerate the dry, acidic soils beneath pine trees. The roots of your hydrangea are having difficulty competing with the tree roots for moisture and nutrients. I'd move the tree out from under the canopy of the pine. Find a spot that provides shade from the hot afternoon sunshine, in soil that drains quickly and replant your hydrangea there. Without competition from tree roots and the overly acidic soils found near pines, your hydrangea should recover.
Best wishes with your garden!
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