says ink cartridge is missing

My Hp Photosmart 6480 printer continues to tell me that the ink cartridge is missing/not detected, so it won't print. For a couple months it would say this and I could restart it a couple times and it would eventually print after waiting like 30 minutes; it would eventual read the cartridge, print a test sheet, and after I hit print it would take 30 minutes before it would suddenly start printing--sometimes it would print really slow. Now it just doesn't work. I've turned the printer on and off, unplug/replugged in, I've removed old cartridges and have used brand new ink cartridges....it just doesn't seem to read that it has cartridges.