The end of that bulb looks pretty dark. How old is it?
If it's less than one year, I'd buy a cheap bulb to test the fixture. If you have the same issue, my money is on a bad ballast. If not, it was just the bulb.
If the bulb is older than a year, I'd just buy a new good bulb as it's probably just reached the end of its useful life.
It is a violation of federal law to use this product in a manner inconsistent with its labelling.