|Sometimes print heads get so bad that they need multiple cleanings.|
On some machines print heads need to be replaced separately every couple of years.
Sometimes ink cartridges need to be replaced due to age, and not just when they are empty.
Sometimes ink gets on the electrical contacts between the ink cartridge and the printer. In this case, they need to be cleaned off with Q-Tips and rubbing alcohol.
Check Canon's website for online manual you can download, look to the troubleshooting section. Also look at their FAQ's on the site for reoccurring problems.
You have to be a little bit crazy to keep you from going insane.