Office JetPro 6978 won't print blue

[KGP51]

Printer stopped printing blue.  I  ran printer maintenance routine then replaced with a new blue cartridge and still did not print blue.  After chatting with HP Agent, received new cartridges from HP InstantInk.  Replaced blue cartridge again but still does not print blue. All other colors print

[sandytechy20]

@KGP51

Welcome to the HP support community.

I understand that the blue cartridge is not working, I am glad to assist you.

Make sure the ink cartridges are properly vented and seated correctly

Check the vents on the top of the ink cartridges. If they are clogged or obstructed, the ink cartridges might not work properly, which can affect print quality.

NOTE:To avoid print quality issues and dried out ink cartridges, do not leave any ink cartridges outside the printer for longer than 30 minutes.

• Turn on the printer, if it is not already turned on.
• Open the ink cartridge access door.
• The carriage moves to the center of the printer.
• 
• Wait until the carriage stops moving before you continue.
• Press the tab inward on the front of the old ink cartridge to release it, and then pull up on the ink cartridge to remove it from the slot.
• 
• Examine the vent area above the HP logo on the top of the ink cartridge.
• 
• If the ink cartridge still has the orange pull-tab attached, pull to remove it.
• 
• If the vent is clogged, use a straight pin to gently remove excess adhesive from the vent.
• 
• Clogged vent
• With a straight pin, gently remove excess adhesive from the vent
• Unclogged vent
• Turn the ink cartridge so that the ink cartridge contacts face downward, and then match the label color with the color dot on the carriage.
• Slide the ink cartridge into its slot, and then gently press the ink cartridge down until it snaps into place.
• 
• Repeat these steps to inspect the vents on each of the ink cartridges.
• Make sure each ink cartridge is firmly installed in its slot. Run your finger along the top of the ink cartridges to feel for any that are protruding, and if any are, press down firmly until each ink cartridge snaps into place.
• Close the ink cartridge access door.
• 

Refer to this HP document for further assistance:- Click here

Keep me posted.

If this helps, please mark the public post as an accepted solution so that it benefits several others.

Cheers.