I've had my HP 722c printer for 10 years +. Never had any problems with cartridges or printing until now. Ink looked like it was running out so replaced black cartridge. Was printing envelope addresses. Was able to print about 15 envelopes and then printer would still run envelopes through fine, but no black ink print. Bought another cartridge thinking it was faulty. Same thing happened with 2nd new cartridge.. Prints for a while, then quits. Took it into shop to have entire thing cleaned -- can find no problems except for severe gumming up that was cleaned. Shop can't get new cartridges to printer either. Any ideas? Love this printer and don't want to part with it.