Hi plaz, Not an expert here, but in my experience( I live in FL), In my experience with my gardenia, especially when the plant is only three months planted I would water it every other day. My guess is you are not watering it enough, once a week for a plant that is not establish yet and blooming needs more water. Since you are in zone 10 I'm guessing it is also hot where you are. I was always of the assumption that flowering plants need more water. Since you checked for pests, try this and see if it makes a difference. I water mine every other day and the plant is full of buds right now. Good Luck!

Here's an article from University of Florida on Gardenias
The whole article is below.

Irrigation
"Watering during dry periods is necessary for healthy gardenias. Moist soil is essential for successful gardenias. Watering is important because it largely controls the number of flower buds that remain on a plant to maturity. If water stress occurs in a heavily budded plant, many buds will fall before opening. Therefore, while the plant is in bud, large variations in soil moisture should be avoided. Use mulch and avoid cultivation around the base of the plant to help maintain adequate moisture"

Here is a link that might be useful: Gardenias (UF)