Diagnosis
The pigment black ink (for documents) clogs more easily than the dye-based colors, especially if you don't print for a week or two. The "cleaning" cycle might not be enough.
How to Fix It
Go into the printer's maintenance menu on the LCD screen: Settings > Device settings > Maintenance > Deep Cleaning. Select the Black ink tank and run it. This uses more ink but forces a stronger flush through the nozzles. After it's done, print a nozzle check pattern (also in the Maintenance menu) to see if the lines are solid.
If That Didn't Work
If deep cleaning doesn't fix it, the print head nozzles might be physically blocked. You can try a manual cleaning. Dampen a paper towel with distilled water, place it in the tray under the parked print head, and close the cover. Leave it for a few hours—the humidity can soften dried ink.
Heads Up
Avoid using third-party "cleaning cartridges" or alcohol. The G7020 print head is permanent and expensive. If manual cleaning fails, a replacement head is the only option, and it costs almost as much as a new printer.