gardenia tree buds
Gardenias are temperamental. Bud drop is a common complaint. All you can do is be sure it's getting the basics it likes:
They want bright light but not direct sunlight.
They like moderate temperatures. (Not too much you can do about that when they are outside.)
They like and need humidity for successful flowering.
They don't like to be moved, but love to be outdoors in the summer.
They need moist soil during active growth/budding/flowering, but can be watered less in the winter. No soggy soil though. Letting the top 1/2" of soil dry between waterings is fine.
Soil should be acid.
Avoid drying drafts indoors.
I am doing all the above except I'm not sure how to give them humidity inside. Some say not to spray the leaves so say to.
One method is to place the potted plant on a tray filled with pebbles, and then fill the tray up to the top of the pebbles with water. The water evaporates up around the plant and creates a little humid "microclimate."
Many folks have given up on gardenias precisely because they are so finicky.