Hello Recluse,
try a couple things first before you spend a much of money buying a new light. There are a couple things that cause a light to be dimmer. I suggest doing this on all of your lights, and they should shine bright light the first time you turned them on. First, make sure the light is unplugged... using WATER ONLY, damp up a paper towel (just wet enough to have the paper towel not dry) using the towel, lightly clean the lenes inside and out...this usually is the cause of a dim light, clean all of your lights so they are very simmilar in the optics. cleaner = brighter. Just do not use any chemicals to clean them, this dammages the lights
If this does not work, the lamp might be going bad. Usually, when ihave a light going out like this, and i have a set of lights, i like to keep the lamps at close to the same lamp hours. The longer the lamp burns (lamp hours) the dimmer it gets... So, i usually replace the lights (leave a white piece of tape on the back of the light with the lamp install date) This way, if i run into a dim light again, i can check the lamp dates on the lights, more often than not they are very different dates with the darker light being the older. Hope this helps you out a bit.