I've never used one of these as printing stuff over the image always seemed a great way to ruin an otherwise fine picture. But a few thoughts...
You say that the LED is "dim". How bright do you think it should be? Keep in mind that the ultrabright LED's we have now are a recent innovation; the original LED's weren't very bright. As for the LED's getting dimmer with age, this is very doubtful. LED's are known for their essentially infinite lifespans. They are, however, sensitive to the supply voltage and small drops in the voltage can cause noticeable dimming of the light.
The only way to really know if it is working properly is to run some film thru it.