6345 keeps saying add toner or out of toner message. open and close door and it will let you make 10 copies then it does it again. customer says this has been going on for a year through multiple toner cartridges. I believe I have changed the drum or drum unit recently also.i looked at the toner add gear in the rear pushed it in and out it seems ok. should I replace it anyway. or could it be something else.