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Check settings in the Canon printer's driver, as to
and any other settings that seem appropriate.
Make sure the right settings for tray selection are set, in your program's Printing settings.
Make sure that in no setting, anywhere, is the setting for Paper Tray or Paper Size or similarly, are set to Auto. Auto is the wrong setting.
If nothing is found wrong with the settings, then turn Off the printer, restart the computer, and only after the computer has started up again, should you turn the printer back On, and try printing again. (Before printing, look at the Print Preview, to make sure the paper size is card-size and as you intend... that can save a lot of paper)
What computer do you have (Windows or Apple) ?
And what program on the computer, are you printing cards from, please?
Get back to me at your convenience. Good night, meanwhile.
Were you using card stock then, when it was working properly?