Answer: Based on your description I am not certain what is happening, but here are some factors to review. These hydrangeas are widely adapted and usually not too difficult to grow, but they do like ample sun, even as much as all day. Shade can reduce flowering. They do well in soil that is evenly moist yet well drained(not sopping wet or saturated, not dried out). Drought can cause smaller blooms. If you need to water, water deeply so it soaks down to the deepest roots. (This is preferable to frequent shallow watering.) Since they seem to be growing well but the flowering is poor, you may be overfertilizing with nitrogen. An annual topdressing with good quality compost plus an annual application of a complete granular or slow release fertilizer such as 10-10-10 should be adequate, especially if combined with a year round layer of organic mulch. This should be in a flat layer over the root zone and about two to three inches deep. If the plant is surrounded by lawn that is being fertilized with a high nitrogen fertilizer, then you would not need to fertilize in addition to that. Flower size can typically be increased over normal size by thinning the plant so it produces fewer, but larger blooms. These bloom on new growth of the season, so any pruning would be done in late winter. Another thing to keep in mind is that older established plants will bloom better than immature plants, too.
It is possible that there is some type of soil problem contributing to a deficiency. I would suggest running some basic soil tests and working with your local county extension to check that major and minor nutrients and the pH are within acceptable levels. I hope this helps you trouble shoot.
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