Answer: I'm not sure what's going on with your hydrangea but it does sound like it is struggling. If it were mine I would dig it up in late fall and inspect the root system. Make sure the soil drains well and that the roots are healthy (brown or rusty colored roots should be pruned away; healthy roots are a creamy white inside). Once you've determined the condition of the soil you can either add organic matter to facilitate drainage, or simply replant your hydrangea and water it in well to settle the soil. Next spring it should perk right up.
The leaf spots are a fungal disease and so some fungicide spraying may be useful, although the season is about over with fall at hand. Your County Extension Office may be able to suggest an appropriate product. If you cannot get such a suggestion I would try something with chlorothalonil in it, such as the product Daconil. Next spring start fertilizing lightly every 4-6 weeks and continue through mid summer. Keep the soil moist but not soggy wet as they don't tolerate such conditions.
Good luck with your garden!
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