I apologize for this delayed reply to your gardening question and hope it is still helpful to you.
Hibiscus needs lots of sun and adequately moist soil. If you allow them to dry out too much or you keep this will too soggy wet they will suffer and start to drop leaves. Based on your description I suspect they may need a little more sun to do their best. Also consider fertilizing them periodically with the soluble plant foods such as Urkel Grow at the label rate.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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