My Tr-J12 has one led that is not as bright as the others , when I put it close to the wall you can see the top one is dimmer
Is the led visually damaged? It has the dome falling off? Not properly centered? Dust or other impurities on the led reflector assembly? Inspected the contacts yet? Have you tried to drive the affected emitter separately?
To little info to be of better help.
I found the problem , there is a crack across the phosphor and only 1/3 of its lit up when I looked at it directly.