ink supply issue.
*check to make sure the ink cartridge itself actually has ink in it. (its wierd, but I've had it happen before...my jv3 registered 3/10 level but cartridge was bone-dry)
could be air in ink line, bad connection between lines>damper>printhead or a cracked manifold (plastic part of printhead that attaches to damper. messing with the dampers sometimes results in this)
....or you simply could have a bad ribbon cable connection. power-down, discharge power, and try to remove/reinsert cables related to that channel.