HP printers have the ability to recognise when the ink cartridges are running low. The printer will display a "low ink" sign and refuse to print until the user replaces the ink. However, the printer can get confused when a refilled cartridge is installed because the cartridge has a chip that the refilled ink does not reset. The only way you can reset an HP 21 black ink cartridge is to trick the printer into accepting it.
Open the lid of your HP printer and remove the HP 21 cartridge by pushing up the clasp that holds it down.
Put a small piece of tape over the top dot in second "line" from the left on the cartridge.
Reinsert the cartridge into the printer and wait for it to print a test page or perform a mechanical alignment, which lasts about 30 seconds.
Remove the HP 21 cartridge again, and put another piece of tape over the top dot in the fifth "line" from the left.
Reinsert the cartridge into the printer and wait for it to print a test page or perform a mechanical alignment.
Remove the HP 21 cartridge again. Pull off the pieces of tape and make sure the contacts are clean, with no glue residue.
Put the cartridge into the printer.