9 out of 10 times, the problem is poor grounds and corroded lamp sockets. Most people don't realize how bright the lights can be when in good order. That means everything from the ground straps on the car, through every bullet connector, the old, dirty rheostat (dimmer knob) and right up to dirty bulbs. Plus, as Tony points out the inside of the cans can get dirty and dull as well. Once everything is in order you will probably double your current output.
Still, many people find even a perfect system to be less than bright enough because they are used to modern cars with better designed instrument illumination. You will never achieve that without serious modification, but there are tricks you can do including LEDs or brighter bulbs. I can't help with that however as I've never used any of that stuff.