The climate is very very hot here. I have recently aquired some gardenias. All had buds and flowers which opened. Now the new buds form beautifully and then drop off before opening. Also one plant's leaves are going brown and dropping.

I noticed a fallen bud on the hibiscus too, which has lots of unopened buds.

Extremely warm temperatures is not good for budding gardenias. Check out this web site;maybe it will be of some help to u:
http://www.thegardenhelper.com/gardenia.htm