Could try following the advice here on how to clean the print heads. http://www.tomshardware.co.uk/faq/id...int-heads.html
I had an Epson printer which I hadn't used for a while suffer from blocked print heads. In the end I bought a new printer. I tried a print head cleaning kit and a new cartridge but nothing worked.
Best thing is to keep using it. Either that or buy a printer that doesn't have built in print heads. I think HP have the print heads as part of the cartridge probably other brands too.