The trouble that I invariably find is that moisture gets into the sleeving where the wires are soldered to the LED. Because the manufacturers of these LEDs are keeping costs to a minimum they use tin plated steel for the diode leads instead of copper. Inevitably the steel corrodes and when the lead has failed you have an open circuit. For a small number of failed diodes it is possible to remove them and solder new ones in, using heat shrink tubing to insulate and protect them. This is a tiresome process though if there are many that need replacing and where there are ten diodes not working (sets using a 31V supply have series strings of ten diodes that are all wired in parallel) it is not always apparent which diode(s) is(are) faulty. The fault I have described does not affect the rest of the lights and because it is an open circuit it just reduces the overall power so it is safe to continue using them if you don’t mind a number of them not working.