Could be larvae of the Hibiscus Sawfly or the caterpillar of the Rose of Sharon Moth aka hibiscus leaf caterpillar. They can defoliate a bush in short order. Japanese beetles will do a lot of damage, but you would have noticed Japanese beetles. Sawfly larvae and moth caterpillars are small skinny green things that blend right in with the plant stems. I don't know that they would eat the flower buds and blossoms though.
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