I thought I needed my HID bulb replaced so I went ahead and had both bulbs replaced, but the driver side HID is still dim. Could it be the actual headlamp? It's not as bright and the passenger side.
Front Driver Headlamp Dim on 2006 Nissan Maxima
If the outside of your projector lens seems dusty or you can't "look inside"; there is dust inside the assembly. I replaced 1 entire headlight, the new one has clear lens and blue/white light. I replaced only the bulb on the other side. On old HID low beam there is a noticeable line across the old headlight lens due to dust/dirt/damage inside the assembly. No help from dealer on how to clean it. Plan to remove headlight and attempt cleaning with compressed air.