An extremely common cause of blossoms falling off hibiscus is insect pests. Tiny insects, usually thrips, feed on their flower buds causing them to fall off prior to blooming. Using an organic insecticide once a week should take care of the problem.
Another common pest problem on hibiscus is gall midge. They lay eggs inside the buds, turning them yellow and eventually causing them to fall. These can all be fixed by an insecticide. If you ever see a yellow bud on your hibiscus, gently pick it off and dispose of them.