Take the cartridges out that are not printing and using a clean tissue, wipe their bases. If you can see colour then they are working. Canon cartridges are ultra-fiddly to seat properly, as I know from experience so be very careful in reseating them.
You could also try deleting the printer and re-installing. You could also have a couple of faulty cartridges and changing them may help. It is very unusual for Canon print heads to clog up or go AWOL.