I've been running into an issue where my DK would be channeling Engulfing Dragonfire and it starts out normally but then its entire visual component disappears. The character is still doing the channeling animation and the damage still ticks but there are just no flames. It seems to happen mostly in dungeons and this may not be relate but I don't think it started happening until I learned Incinerate and started using it in conjunction with Engulfing Dragonfire.