The C/21 uses a power supply regulator that can drift in output voltage and it's set by a resistive voltage divider at a pin of the regulator chip.
I've seen them drift. I've set my own C/21 to 14.5Vdc output and it runs almost 40W output power.

At the original 12Vdc, it doesn't. But it's easy to change that.
Of course your particular issue could be something completely different, but I'd measure the power supply voltage first.