Much as I love Canon printers, I've just taken a serious dislike to the way that they present their manuals. Instead of being able to download a pdf file (or perhaps 1 or 2Mb), I've had to download a 200Mb zip file, unzip it and install a program on my computer in order to (try to) read the manual. However it turns out that the program isn't compatible with Windows 7 anyway, so I still can't view it anyway! Grrr!
However, having looked at the on-screen manual for my own Canon printer (MG5350), it seems that (if yours is somewhat similar) you might find something useful there:
Go to Start > All Programs and click on the folder which says something like 'Canon MP220 series Manual'. Then click on the sub-entry for the manual itself. Look in the contents list for 'Troubleshooting' and click on it. Then do the same for 'Problems with scanning'.
With my own printer's on-screen manual there's a section about print being too light. Hopefully you'll find something similar and be able to make use of it.