I have a Hampton Bay Bathroom wall mount Florescent..it requiresF18DBX type bulb which is a compact florescent with a specific plug in type base...very specific. The light works intermittently but when it lights it lights well and consistently...sometime at the flick of the switch but often I'll just leave the switch on and sometime in the future unknown it lights.

I looked for a starter (old school can) not there. The ballast does not look replaceable.

Is this a "Throw Away" fixture ? I can't find a Hampton Bay website with troubleshooting...it only goes to Home Depot

 

You may want to check to see if your wiring method is grounded. Fluourescents do not work well on ungrounded circuits.

Your bulb may be starting to fail also.