Is the bulb seated correctly inside the housing? Either it's not seated correctly, the reflector thing in the center is broken or off, or the headlamp is way out of aim or not aligned properly on the car.
I would replace the bulb or swap one in from the other side as well, to make sure there's no hot spot on the bulb that might be screwing up light output.
If all else fails, you might need to replace the headlamp altogether.
