How to grow houseplant gardenia
There is no one answer to each of those symptoms.
Gardenias are not an easy houseplant. They often drop buds and have yellow leaves. A common cause of bud drop is a drop in temperature (drafts or too chose to window in winter). Leaf yellowing is often soil that is not acidic enough or low in nutrients (needs fertilizing.) However, there are many other causes of these same symptoms.
Read through the following webpage: http://www.clemson.edu/extension/hgic/pests/plant_pests/shrubs/hgic2058.html
Try to determine how your gardenia's culture varies from the ideal. Also, read through the causes of each symptom.