*****Switch the bulbs out from one side to the other and see what difference it makes*****
This has happened to me a few times and i have dived deep into it trying to figure it out, when it was always just a bad bulb.
Switch the 2 around, it won't cost you anything and see if it is then dimmer on the other side. If it is, it's a bad bulb with a weak connection.
I put a new bulb into my tracker once and it was really dim and it took forever for me to figure out that it was just defective and the connections of the wires inside the actual bulb were not very strong.