There is a another possible issue. The ground for the light socket may be hooked up to the wrong cavity. As you look at the rear of the light, the ground connection is to the right. I have seen them hooked up to the center post. The light will burn, but it will be very dim. Probably a long shot, but I have found this problem.

Also, depending on how the lights are wired, there are many spots that bad connections can occur. If salvage yard parts, such as the light switch and dimmer, were used they may be getting bad or worn.

Start at the main supply with a voltmeter and check at each switch. Look for voltage drop across each one.