HP Officejet 4620 not printing black from new cartridge

The church secretary has an HP Officejet 4620 that she replaced cartridges with brand new cartridges.

Test page : no black, all other colors are fine.

The ink did come out when I touched the pad, so I know it's not dry. We also tested it in another printer in the school and the cartridge worked.

Here's what I've tried:

1. Running the machine's own cleaning procedure x3.
2. Aligning the print heads.
3. Manually cleaning the print heads on the machine (unplugged) with a towel slid under the print heads, and swabs of rubbing alcohol. Repeated steps until the swabs came away clear and there was no further ink coming out on the towel.

If I could physically remove the block with the printer heads, maybe I could get more out. Does anyone know of a schematic online to allow me to safely remove the print head carriage?