I built a document that required numbered headings (1., 1.1, 1.1.1, 1.1.1.1, 2, 2.1, 2.1.1, 2.1.2, etc.). Everything works fine except Heading Level 3. Where the number and the space following it should be, there is a black "box", for want of a better description. The number is, apparently, under the black box, because lower level headings are appropriately numbered (and visible). I haven't been able to locate anything that instructs the numbering to do that and I checked the border style - there is no border of shading associated with the heading level.

Does anyone have any idea what could be causing it?

Thanks.