Black is printing dull - help me!

For awhile, my product labels were being printed from Illustrator with a rich, deep black. At some point, something happened and now the black is printing very dull. It's been like this for months - I've contacted Brother to troubleshoot and they said the problem was not my printer, but rather a setting in Illustrator. I've tried contacting Adobe but can't get through to them (on hold forever).

I've messed around with settings on both my printer and Illustrator and have already tried a factory reset on both. Nothing seems to work. What do I need to do in order to get my labels printing rich black again? Someone, please help me! TIA