Hydrangeas - brown leaves, bugs etc

Hello all,
For a few weeks my small potted hydrangeas have taken a turn for the worst. Their leaves have started browning; some turning 'crisp', and some turning 'soggy' - so I'm not clear whether I'm overwatering or underwatering. I water them a little every day, the pots drain well, and they're in a part of the garden that gets about 2 hours of sun a day (on a good day), so they're not being scorched.
This morning I took a closer look, and noticed that inside new buds and growing leaves, there are green and white flies, and that some of the stems also seem to have black spots on them.
Are these bugs likely to be why the plants are looking so miserable? And which sort of insecticide should I be using? Should I be watering more or less? And should I move them to a completely shady part of the garden?
Any advice gratefully received

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