My CANON S750 has started playing up. The colour started getting worse on prints although they weren't quite empty - so I replaced the ink cartridges anyway. Now it will only print in black and white not colour. Where the colour should be on a printed page, there is nothing. I've tried the usual cleaning and nozzle checks through printer utilites and checked that the cartridges are seated properly. It is only about 18months old. Any ideas?