99% of all poor print output from a Canon printer is usually down to the cartridges not providing enough ink to give good coverage and only 1% is caused by the print head.
There are many reasons why you get poor ink flow (Ink starvation) the main ones can be due to the quality of inks used and the method you use to refill and maintaining the cartridges.
At the very first sign of poor output you should investigate all of the cartridges and not just the one you think is having the problem, often it’s not what or where you think the problem is, you should avoid the temptation of removing the print head and making your problems bigger.
The most important thing of all is to remember not to continue to print while there is clear evidence of poor output and use the nozzle check as your only guide till the problem has been rectified completely.
When you think you have the situation under control and resolved then run a couple of sheets with one inch colour strips on them to check for proper ink flow, BUT if there is any signs that the original issue still exists then STOP the printer immediately by pressing the Resume button, reinvestigate the whole problem again.
So ends the lesson..