Take a look at your nozzle check to see if you are missing black nozzles. If you are, a good cleaning is in order. You may also need to replace some maintenance parts such as the cap top and dampers. Check the bottom of the head for any ink build up that may be causing the seal between the capo top and head to break. Do a head soak and hard cleaning. This should recover any recoverable nozzles. If everything looks good and you can't get the nozzles back, your last resort is to flush the head manually with a syringe and cleaning solution. If you can't recover them that way, you need a new head.
If the nozzles aren't missing in the first place I would replace the black dampers and see if that helps at all. If not, it's most likely a bad head especially if it is older than 3 years.
