I have a small hibisucs and the buds fall off just before they open.

How old is the hibiscus? Is it in the pot or the ground?

It is most likely caused by an insect infestation,apply a systemic insect control and spray the plant down with horticultural oil.If that does not help you will have to re-pot it again as the insects lay eggs in the soil and the problem will continue.

I'm not sure it is bugs since you didn't mention them. Try adding Epsom Salt to the pot for the magnesium. Make sure it is getting enough moisture and let it dry (not completely) before watering again. Too much water is just as much of a problem as too little. What kind of soil did you use to pot it?

That should be a complete mix. It won't hurt to add Epsom Salt though. 2 tablespoons per pot.