Certified GKH Gardening Expert
They can be a little picky about their watering. Usually, overwatering causes this more than underwatering. Sometimes this can be do to lack of nutrients, but the way its forming doesn't fit the bill.
Make sure that these have plenty of time for the soil to dry out between waterings. I would also apply a half cup of DOLOMITIC LIME and a teaspoon of WETTABLE SULFUR to mitigate any infection trying to set in. Do this once per year for more protection.
This article will help you to care for Hibiscus: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-container-care.htm