Have had a blocked head for a while, and no amount of "deep cleaning" (from the tools menu) would sort it. So googled it, and some chap on a geek forum had the same problem with his Canon and suggested taking out the print head (I didn't even know it did come out) and then - "here comes the science bit" - chuck it in a bowl of soapy (w.up. liquid) water overnight, dry thoroughly, put back in, run a couple of "deep cleans" and JOB DONE.