Are you watering the top of the plant, including the leaves, or watering the soil? If you are getting the leaves wet when watering, it can cause problems as can poor drainage. Poor drainage will cause both the leaves to yellow and buds to drop off. Another cause of yellow leaves is soil with a high pH. Too high, and the gardenia isn’t able to take in nutrients including magnesium and iron, resulting in yellow leaves. Buds dropping off can also be caused by low humidity. It’s easily solved by placing the container on a tray of pebbles with water. One more thought, if the evenings have been cool, the buds will react by dropping off. I hope these tips help and you are able to get your gardenia healthy again.
Thanks for writing!