I had that problem several years ago, but I can't remember how I cured it. The solution was not spraying the foliage but adding the cure to the soil. I'm sorry I can't be of more help, but maybe my reply will jog someone else's memory.

Here is a link that might be useful: Hawaiian Sunset

Thrips are usually the culprit with bud drop. Use spinosad to control thrips on your hibiscus.

felipo, I remember now that my problem was caused by something called a gall midge. The solution was to treat the soil with a granular compound with imidicloprid as the active ingredient. Remove from the soil any fallen buds, pick the yellowed buds off your plant everyday and get rid of them.

Here is a link that might be useful: Gall midge control

Thanks so much for your help. Will try imidicloprid to see if it takes care of the problem. We have had torrential rains here in south Florida and perhaps that has also played a role.