If you are setup CMYK x 2 then the cyan channels are connected and so when one side has an issue, it affects the other side as well. If the dampers didn't fix the issue, the connector that they connect to could be failing. I would change those along with the o-ring that seals the connector with the ink line.
If that doesn't fix the issue you are going to have to start looking deeper in the machine and more specifically, the ink lines and valves that are located behind the ink cartridge holder. The valves can clog or fail causing ink flow issues. One way to test this is to look at which cartridge the machine is currently pulling from and remove it. Then run a cleaning cycle and see if the cyan comes back. If it does, that means the cartridge you pulled out is either empty and not flagging the machine properly or the valve is clogged and need to be cleared out. You can also visually watch the valves during a cleaning. only 4 should open during the cleaning cycle. If you only see 3 open or one looks like it only half opened, you might have a burnt out solenoid and the valve just can't open. I have never seen a valve fail other than being clogged.
If none of that works, you either have an air leak somewhere in the system or possible internal damage of the print head. Since the cyan channels are not directly next to each other on the head however, that is less likely but not out of the question.
Also, just to be thorough, make sure the cap is nice and clean, the pump tubes aren't clogged and the bottom of the head doesn't have any ink build up or debris causing it to no seal properly with the cap top. .
