I haven't used my printer for a while and both cartridges needed changing, both colour and black. The colour one is now printing fine, but the black is not. I ahve taken it out and replaced it, pressed it down a bit, checked with my finger that there is ink on the pad - none of these seem to have solved the problem. Woudl be grateful for any advice, please. I ahve a lot of important letters to print, and I really need it! Thanks.