The troubleshooting steps in the document here may help resolve cyan not printing on your Officejet Pro 6978.
If the standard troubleshooting did not resolve cyan not printing I would suggest removing the cyan cartridge. Turn it over and examine the area around the ink port. There are four orange plastic spots, left over from the protective cap. Make sure these are flush with the cartridge body, if not carefully trim them flush. If the plastic nubs protrude they can prevent the cartridge from seating properly. Before replacing the cartridge in the printer dribble a few (maybe four) drops of hot water into the ink port in the printer and two drops into the ink port on the cartridge then snap the cartridge back in position.
It may also help to manually clean the printhead as described in the post here.
I have also seen cases where leaving the printer overnight after the cleaning cycles allows printing to recover.
If none of the above restores proper printing your printer will likely require repair or replacement.