It's generally down to condensation causing corrosion on the led legs. About half of my indicators are out and two brake LEDs. It should be possible to just resolder them to fix the poor connection, but if they get too hot, the LEDs break. This has happened with one of the brake LEDs and about 8 of the indicators
I am more than up for full replacement, although that does mean removing and replacing about 60 individual LEDS and the removed ones are likely to be useless. knowing me, I would probably want to do both sides too so they matched!