Answer: If it looks healthy and happy but isn't blooming, it might be that it is freezing down to ground level each winter. Hydrangea macrophylla, the big leaf hydrangea, flowers on new shoots which develop on old wood. Without old wood, your plant will grow wonderfully well, but will not flower. If this is the problem you might be able to protect the old canes from freezing completely by insulating them from the winter weather. One way to do this is to cut the canes down to about 18", then mound pine boughs or leaves or even compost over the top of the plant. In the spring, brush away the insulating material and new shoots should develop from the old wood. These shoots should flower for you.
Best wishes with your hydrangea!
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