Xerox DC240 leaves toning on coated stock and no toning on uncoated

We're puzzled by this. When we print on 20-60# uncoated paper there is no tinting or toning of tiny specks of toner in the background of the sheet.

When printing on coated 80-100# 2/sided gloss we get a faint but noticable hue in the background of the image.

A parts supplier suggested that the black developer should be changed.

Any insight or suggestions are very welcome.