I repalced both of my lights at the same time, plus I had the comptuer module, which could make a difference in what you are perceiving vs. what happened with me. However, what I can say is both of my lights worked, but one bulb was dimmer than the other. I purcahsed a pair of brand new bulbs on eBay (Phillips DS2 if I recall as that is what the manual stated it used). Made a significant difference. If you pull your bulbs and see yellow spotting ont he glass, that is the metallic salt fromt he Xenon gas (if I cam describing this correctly) and indicates your bulbs need replacement.
From previous experiance with Xenon type lighting, I can tell you that it may not fire up properly if the bulb is bad, the ballast is bad - or going bad, you have poor electrical connections, etc. But I'd start with a look at the bulbs. These are what I bought on eBay - except I paid $65 each at that time ...
New Philips D2S HID Xenon Bulb BMW Audi Acura Saab | eBay
Make sure you tell him what catr they are for and stress the DS2 (if that is what is in your lights - you can check to make sure).