Type of bulbs?? Halogen or Xenons??
Assuming low beams, try swapping the bulbs from right to left. If the problem follows then it's the bulb. If it doesn't follow and it's a halogen, then the lighting module, wiring or the headlight switch could be the issue.
If its a Xenon bulb then try swapping the ignitor. If it follows with the ignitor, replace the ignitor. If swapping the bulb and or igniter has no effect, you are looking at the lighting module, wiring, or the switch....