Diagnosis
This is almost always clogged ink nozzles in the printhead. The ink has dried in the microscopic jets, blocking the flow. The built-in cleaning cycles might not be strong enough.
How to Fix It
Run the printer's deep cleaning utility. On the control panel, press the 'Settings' button, navigate to 'Maintenance', then 'Clean Print Head'. Choose 'Deep Cleaning'. This uses more ink and takes longer but can clear moderate clogs. Print a nozzle check pattern afterwards (also in Maintenance) to see if it improved.
If That Didn't Work
For stubborn clogs, you need to manually clean the printhead. You'll have to remove the ink tanks, then the printhead itself. Dampen a lint-free cloth with distilled water and gently dab (don't rub) the nozzle area on the bottom of the printhead. Let it sit for a few minutes, reinstall, and run a cleaning cycle.
Heads Up
Don't use tap water, alcohol, or window cleaner on the printhead—they can damage it. And be prepared to waste a non-trivial amount of ink from your INKvestment tanks during deep cleaning cycles.