What make/model of fans? Are the LED electrical contacts exposed or at least accessible so you can use a multimeter to check the circuit? Do you have a multimeter?
Is the fan on a fan controller or motherboard controlled header that varies the speed? The LEDs are "probably" in series with a resistor and a voltage too low could cause them not to light up at all, not just be dimmer similar to how the fans might get slower with a lower (average if PWM) voltage. Then again I expect you would have noticed that on a new build, before two weeks passed.