Earlier, someone mentioned the possibility that the bulbs may have been replaced with ones of the wrong voltage-higher voltage bulbs would indeed glow more dimly). You've noticed that there are some bulbs that appear brighter. Would your manager allow you to swap the bulbs from the brighter buttons into the dimmer ones? This should help rule out the bulbs themselves as the cause of your trouble. You should be able to do this without having to take the board off-line.
Maybe not likely, but heat can cause some plastics to darken, check the bulb covers for discoloration or cloudiness.
You know, the more I look into this, the more I'm wondering whether it's a question of the bulbs as you speculated. I just removed the covers from two of the yellow "Off" buttons along the bottom of the board. One would think they'd all be identical--they're the same type of button. However, one of the LED bulbs (as I look at it without the cover) is clearly, discernibly dimmer than the other one.
I guess the next question is--how do I know what bulbs to replace these dim ones with, specifications-wise.....and how would I replace them. LOL. I may have to take a class in electrical engineering....