Hibiscus dropping buds is a sign of stress. I am not sure what kind of stress the plant is in, but that is for you to figure out :) In my mind, indoor plants maybe getting too less light and/or being pestered by insects like spider mites. If the hibiscus are more than a year old, they will bud and flower under the right conditions. Please check your fertilizer to ensure that it is not hurting the hibiscus, and also that it is applied at the correct dosage. Check your watering habits also, because the transpiration through leaves is different indoors from outdoors. A good place to do research on growing hibiscus is Charles Black's website. BTW, I am in no way related or profit from Charles Black or his business.

Good luck.

The list of things that can cause buds to abort is long. Insects, disease, too much P in your fertilizer, root congestion, over/under-watering - which usually goes along with a poor soil choice, heat stress, a high level of salts in the soil .....

The main two things that most affected my plants were P levels and root congestion. Potted hibiscus do best with a full repot yearly. Full repots include bare rooting and root pruning the plant, and a change of soil, preferably something appropriate. Stay away from bloom-booster fertilizers. Even 1:1:1 ratio fertilizers like the popular 20-20-20 and controlled release 14-14-14 supply much too much P in relation to N for the plant to be as happy as it could be.

Al