My HP Deskjet 2135 says 'Ink Levels Low' but I just replaced the cartridges. What should I do?

Ensure the new cartridges are properly installed and seated correctly. Remove and re-install the cartridges, making sure they click into place. Clean the printhead using the printer's built-in maintenance utility (accessible through the HP Smart app or printer software). Sometimes, the printer needs to 'learn' the new cartridges; a power cycle (turn off, wait 60 seconds, turn on) can help.