1250C Laser printer printing grey instead of black

Hi. I have a 1250 c laser printer with fairly full cartriges. All of a sudden, it prints text grey instead of black. It prints black graphics fine. I read on another support forum that you can reset the defaults in toolbox and that would fix it. It actually did work. However, it's doing it again now and resetting the defaults no longer fixes the problem. Any help would be much appreciated!

Thanks.

Kat