There should be some sort of manual adjustment on the light is what the previous poster meant. I have a set of D2S bulbs that came with my car that has xenons, never fitted thembut supposedly from reading they are an upgrade from the original bulbs which either loose intensity over time or are weak from day one. Might be worth a try, I think the bulbs are from Perfekt Lighting (ebay I think but a trader on another forum whom I bought LED reverse bulbs off which were a good upgrade)
It could be the same scenario as old flourescents, the bulbs just loose some output over time? If you were really bothered I think a set of replacement bulbs could be worth a try.