There are a lot of possible reasons why gardenia buds are dropping. Since gardenia is quite sensitive any changes in its environment may cause it to readjust and affect bud formation.
The possible causes and solutions include :
Infrequent watering: Try to keep the soil evenly moist and not wet or soggy. Otherwise, gardenias will have yellow leaves or buds falling off. Too much direct sunlight: Put your gardenias in a place where they can only get indirect sunlight. You can also put a net or a sheet of thin fabric to block excess sunlight.  Low humidity: Gardenias need an adequate amount of humidity. So, when the air is dry, place a humidifier near the gardenia. Soil adaption issues: Try not to move your gardenias from one pot to another since they can get stressed when place in a new environment Nutrient deficiency: Probably the first thing that comes to mind when you see a bud drop is a nutrient deficiency. Apply a fertilizer suitable for gardenias, follow the labels, and avoid overfertilization.  Any drastic change in temperatures: During the day, gardenias grow best at 64 to 69.8 °F and 59 to 64 °F at night. If the weather is beyond the ideal temperature for your gardenias, you can place them in a temperature-controlled environment to boost bud formation.