Try following the steps in the link below
Failing that, if the printer is out of warranty then it might be worth soaking the printhead as described below....
1) Get a pan and put 1/2" of very hot (not boiling) water in the bottom.
2) Next remove all the ink cartridges, then remove the printhead assembly.
3) Gently put the printhead nozzles down in the water and let it soak for five minutes.
4) Remove the printhead and gently dry the electrical contacts.
5) Put the printhead back in the printer and then reinstall the ink cartridges.
I am speaking for myself and not for HP. Twitter: @Ciara_B_HP