Dell 5110cn - Black toner issue

Dell 5110cn is not getting black toner to page. New toner modules have been installed but page is very faint, especially through the middle. I have checked the fuser, imaging drum, transfer roller, all seem to be working and have checked them in another 5110cn. Do see a pile of black toner in lower left corner below where the imaging drum slides in. Cleaning the developer has made the output almost blank. Color output is fine.

How is toner supplied to the imaging drum? Could there be some sort of leak or blockage? How can I test?

Have output examples and pictures but don't see how to include these here.