There are 4 possibilities.
1. The bulb is bad. Try swapping bulbs around in the fixture and see if the problem follows the bulb or stays at the socket.
2. The capacitor is bad if it's a magnetic ballast. If it's not the bulb, this is your most likely culprit. I've seen many-a fixture behave like this that had bad caps.
3. The ignitor is bad if it's an HQI (or Pulse Start) system. Since the bulb starts to light, I'd say that this is unlikely.
4. The ballast is bad if it's an electronic ballast.
Start by swapping the bulbs and let us know what happens. If the problem stays at the socket, then we're gonna have to dig deeper. If it follows the bulb, get a new bulb.
