HP Photosmart 7450 colour cartridge HP 57?

My HP Photosmart 7450 gives the message "The print cartridge on the left side of the cradle is not compatable with this printer, "Please remove and reinsert". This has just started today 1st October 2009. The cartridge, an HP compatable, has been in the printer for several months and has only been used a couple of times. When an empty HP cartridge is inserted, the message does not appear. Any suggestions please.