I have the exact same problem, some colors not printing even with new cartdriges (sligthly different printer, mine is the C7280, but doesn’t seem to be much difference). I have dissasembled it completely and reached the print head. It is not a piece of cake, but it is not impossible either, it just required patience. Cleaned the prnit head with IPA, and reassembled. Good news is printer works as before, no harm done. Bad news is, that didn’t solve the issue and some colors still don’t print. I’m assuming there’s some dried ink somewhere.