The only thing I can think of is that there is a subtle difference in the old and new lights - filament resistance, specifically. If the new ones are higher resistance, then the old ones wouldn't be getting the current they're designed for, and would be dimmer.
If you still have the old bulbs, try putting them back in. Otherwise, see if the new bulbs are brighter, and if so, replace all the old ones with new ones. (The new ones may dim as you replace old ones, as you'd be increasing the total resistance of the circuit.)