About a couple of years ago I made up some edge lighting for a meter in a pal's small recording studio. To replace the tiny 24v tungsten which were forever blowing. Used 9 diffused 5mm white LEDs - 3 each in series and one series resistor per chain of three setting the current at something like 3 mA which was plenty bright enough - and the end result was very even illumination. Much better than the tungsten.
Very unlikely to be the PS as that is used for other indicators too - which were ok.
Checking things here on my own PS, it's the LEDs themselves which have all gone low output. Some more than others - but all well down on the spares I have from the same batch - bought off Ebay. And rather bluer than the new spares. The series resistors are all in perfect condition - no signs of ever even having got warm.
I've had the odd LED fail - but never anything like this. Any explanation?