Brother DCP-4020C - Manual (Page 212)

TROUBLESHOOTING AND ROUTINE MAINTENANCE 15 - 11
If you still see missing ink in this Print Quality Check Sheet, repeat
the cleaning and test print procedures at least five times. If ink is still
missing after the fifth cleaning and test print, replace the ink cartridge
for the clogged colour. (Perhaps the ink cartridge was in your DCP
over six months or the expiration date written on the cartridge
package has passed. Or, the ink may not have been stored properly
before use.)
After replacing the ink cartridge, check it by printing another Print
Quality Check Sheet. If the problem still exists, repeat the cleaning
and test print procedures at least five times for the new ink cartridge.
If ink is still missing, call your Brother dealer or Brother for service.
Caution
Do NOT touch the print head. Touching the print heads may cause
permanent damage and may void the print head’s warranty.