PGBK Cartridge won't print.

Similar to all previous posts, my Canon MP550 won't print anything with the black PGBK cartridge, all other cartridges are printing fine. I have run nozzle clean, deep clean, removed, washed out and remounted print head, re-ran nozzle clean & deep clean, Still nothing is printed by the black PGBK cartridge, all other cartridges are printing fine. What next??? Please help!!!