There are a variety of reasons why buds fall off of a gardenia plant, but the most common cause is a change in location. Improper watering habits can also cause gardenia buds to fall off the plant.
Gardenias are very sensitive to disturbances and don't do well when they are transplanted or even touched. To prevent gardenias from losing their buds, the plants should be left alone in one spot. Another reason why buds sometimes drop off is due to lack of moisture. When the plants aren't watered properly, the buds will rot and fall off. The soil they are planted in should be kept moist, and the air should be humid to prevent buds from dropping.