There are a number of things that could be the culprit, such as thrips, caterpillars or nematodes. It could be a deficiency or overabundance of nutrients, or it could be caused by poor drainage, too much water or too little water. Examine your plant and the area for any of the above. If none of the above exists, be comforted in that it is the hibiscus' nature to prematurely drop buds, especially the doubles. Some varieties will do well for only a portion of the year. During excessively hot and dry weather, bud drop is often caused because the plant is not able to absorb the water it needs to compensate for the water lost through transpiration.