The Canon Pixma Pro 100 has TWO (that is 2) paper trays toward the back of the printer. This is not at all obvious to someone who is setting up a new printer.
For aligning the print head you need to insert two sheets of paper into the "rear paper tray", which is actually IN FRONT OF the MANUAL FEED PAPER TRAY. That is, the manual feed paper tray is BEHIND the "rear paper tray".
The REAR PAPER TRAY cover is on the TOP of the printer. Lift that cover UP to expose the "rear paper tray".
The MANUAL FEED PAPER TRAY cover is at the BACK of the printer. Pull that cover OUTWARD to expose the manual feed paper tray.
Again, for aligning the print heads one must insert paper into the REAR PAPER TRAY (accessed at the TOP of the printer), not the manual feed paper tray (accessed at the REAR of the printer).
One might intuitively assume that the manual feed paper tray is the "rear paper tray" because the manual feed paper tray is at the "rear" of the printer, but that assumption would be incorrect. The "rear paper tray" is (to a new user) hidden under the lid on top of the printer.
