I agree with Garrett - could be just worn bulbs. Assuming that your wiring and chargings system are up to snuff - some OEM bulbs will start to dim in output in as little as two years. Most don't notice it because it is so gradual.
New OEM bulbs will get you back some of that light - Sylvania Silver Stars are also said to be a good choice in aftermarket. DOT legal and same wattage as OEM - just runs at a slightly higher color temperature (not blue bulbs - closer to Halogens). PIAA Extreme White and Super White bulbs are also very bright bulbs (higher wattage)that cost about double the price of the Sylvania Silver Stars -but are also reputed to burn out quickly (less than 12 months).
Long life - can't beat OEM, for more light - I'd suggest Sliver Stars (about the same price as OEM).