MG8120 Printing with any Media type other than "Plain Paper" gives a washed picture

[T16]

I'm trying to print some monochromatic images on a "Photo Paper Plus Glossy II" paper using a Pixma MG8120 printer.

However, under the Printer's Preferences, if I set the Printer's Media Type to "Photo Paper Glossy II" (to match with the paper I'll use to print with), I get a washed out shade of black.

Nevertheless, if I set the Media Type to be "Plain Paper" I do get a real black print... Why would the "Plain Paper" Media Type give an accurate shade of black and the "Photo Paper Glossy II" won't? I also tested changing the Media Type to "Glossy Photo Paper" and the black print is also washed out. Playing with the Intensity and Contrast doesn't make it darker.

Any ideas? Please refer to the image for a visual example of my problem.

They are black squares printed under different Media Types.