Hello, I just changed the 9" circline flouescent bulb on my hampton bay fan. The bulb does not stay on, it flashes on briefly and then goes off.. Home depot says they can't test the bulb because they don't have the connector switch onsite. I guess I could get another bulb from them and try it.
Could it also be the ballast that is not working ?

With Flourescents, I try the bulb first, usually the cheapest and fastest, if second bulb doesn't work, you got a bad bulb, not full voltage to ballast, not a good ground or a bad ballast.
Or try bulb in a known fixture or fan.