DIAGNOSIS The printhead nozzles are clogged with dried ink. This is a maintenance issue, not a hardware failure.
FIX On the printer's control panel, press the Setup button. Navigate to Maintenance > Deep Cleaning. Select the color that's streaking, or run it for all colors. This uses a significant amount of ink. Print a nozzle check pattern afterwards (Maintenance > Print Nozzle Check) to verify the fix.
IF THAT DIDN'T WORK The ink may be old or the wrong type. Ensure you are using genuine Canon GI-790 series cartridges. Third-party ink can dry faster and clog.
HEADS UP Do not run deep cleaning more than twice in a row. It floods the waste ink pad. If two cycles don't work, you need manual cleaning or a new printhead.