Answer: Based on your description I am not certain what the problem would be, however it could be due to lack of sun or other cultural conditions, or to improper pruning.
Hydrangeas do best in morning sun or in bright dappled shade. In too much shade, they bloom poorly.
They also need adequate soil moisture. The soil should be consistently damp like a wrung out sponge, not sopping wet and not dried out. Using an organic mulch several inches thick year round can help keep the soil more evenly moist.
They also need fairly rich soil. An annual top dressing with compost and/or an all purpose slow release or granular fertilizer should be adequate. Read and follow the label directions for how much to use. The organic mulch will also help feed the soil as it breaks down over time.
Most hydrangeas bloom on old wood that grew the year before. For this reason, any pruning should be done right after it blooms. You would thin the plant and possibly cut it back at that time if needed to control size. Pruning in winter or spring would remove flowering wood for the coming season.
I hope this helps you trouble shoot.
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