U were right i think. I put 2 9V batteries betwein the main power source and the LEDs and all of them worked.This may seem to be a silly question but, have you checked the supply voltage?
It appears that the LEDs may be in series groups which are then connected in parallel but with a common resistor. With this type of (poor) arrangement, as the supply voltage falls, some LEDs will not light which gives the impression that they have failed.
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