I am responding because no one else has and I know what it is like to loose a beautiful plant. I also have to qualify this response by telling you I do not have any hibiscuses planted in my garden (Montreal, Quebec gets far too cold). And, this is the first year I've started buying these plants.

Anyway, I do have a beautiful yellow hibiscus tree in my house and from experience I can tell you that when leaves started to turn yellow and drop off, and, the flower buds seem to drop off just by looking at them, I did not know if it was the heat or the watering. Read that these plants can take the heat and full sun (as long as there is a 2 hour respite from full sun), so must have been the watering. Currently, am not watering as much and only when dirt is slightly dry versus slightly wet (the slightly dry is subjective, of course).

Good luck.

I had this same problem -- little bugs were eating the bud stems causing the unopened bud to turn yellow and drop onto the leaf. Spray the new bud area with pesticisde or dust with sevin -- it will clear right up.