I have a DIY LED strip w/ 12 blue LED's. Today, 2 wont light, 1 is so dim you can barely tell its on at all and the rest are so dim they wont light up the tank at all but you can see them glow. None are bright anymore.....
They light right up when plugged in. They dont turn off right away though....takes a few seconds and they fad out instead of turning off instantly.
They will always fade out because of the AC-DC converter in the driver.
Start with what Tom mentioned. It could be that the thermal paste has silver in it and is contacting a pad and the heat sink. It also could be that you just have a couple bad LEDs. Before you plugged everything up, did you set the internal pot on the driver to the volt/current loads you needed for the number of LEDs driven?