Fixtures not dimming with fader.

Our Colorsource 20 board has been operating fine until recently. In playback mode our fixtures (Elation Fresnel KL8's and KL6's)  will turn on with the assigned playback fader, but 6 of our 15 fixtures will not dim or turn off in the same manner. They will not turn off nor dim unless I go into the individual channel mode. And when I go into the channel mode none of the faders indicate that those particular fixtures are even on. I have to use the channel faders to turn them on and only then will the fixtures turn off. Any ideas what could have happened? Would a power outage or anything like that cause the board or the fixtures to behave this way?