When you installed the printer did you go through the align procedure using the matte paper that they provide in the box?
"Align the Print Head. Canon has improved the print head alignment process from the MP620. There is a pack of two alignment sheets (Matte Photo Paper MP-101, a heavy paper with a notched corner to correctly orient the paper in the printer).
Load one sheet of the paper and follow the directions on the LCD (you know, Press OK) and in a few minutes, the printer will both print and scan the alignment bars."
This quotation from this website for the MP640 click here