Premature bud drop is often a problem with hibiscus. Some varieties, especially some doubles, are prone to bud drop. Other varieties bloom well during one period of the year and regularly drop their buds at other times. Bud drop can also be caused by insects such as thrips and midges. Nematodes, nutritional deficiencies, over-fertilization, and environmental factors such as poor drainage and excessive water, drought, or salt spray can also cause flower buds to drop.

Additional Resources on Hibisbus

http://edis.ifas.ufl.edu/MG020