It is a common problem for people who grow gardenia, particularly in containers.  Some of the causes are:

1) Change in location.  Gardenias do not like to be disturbed. They are extremely sensitive to being moved or even touched. Try to keep the plant in the same location - always.

2) Improper watering. Gardenias like to be kept moist. If they are allowed to dry out too much, they will respond by dropping their buds.  Water every day (as needed) and at the same time of day if possible. 

3) Overly dry air, causes the buds to die and abort.  If the buds are aborting before being fully formed then low humidity might be the problem.  In the state of Arizona or Arid-Zona (Arid Zone) this is moist likely the problem.  Try: a) keeping the soil evenly moist; b), mulch the soil surface of the pot with sphagnum moss to hold in moisture & to increase acid levels in the soil; c) place the plant pot into a larger container to achieve ½-1 inch of air space between the two containers; and d) Use a deep drain tray with 1-2 inches of gravel placed in the tray with the containers sitting on top of the gravel and keep the gravel moist.  These steps will help keep humidity levels high.

One other cause is thrips.  Thrips are minute rasping insects that will scar tender bud tissue to extract water and nutrients.  If the flower buds are fully formed, then drop before opening your problem might be thrips.  Checking is easy.  Take a white sheet of paper and break a few of the dropped buds onto the paper in tiny pieces.  Thrips will begin running all over the paper and they can be easily seen.  Thrips are generally black, brown, gray or yellow. They will be about 1/16 inch long and as big around a a strand of human hair.  The white paper makes it easy to see them.  Photos are attached to assist you in identification of thrips.