WALLS NOT DISPLAYING IN PLAN VIEW

Problem:

Some walls in a Floor Plan view are not visible, or appear to have no thickness or fill.

Yet the same walls do appear in other views of the same level, or in a newly Duplicated view (without Detailing).

Cause 1:

There is a Plan Region in the view.

Solution:

Delete any Plan Regions and recreate them.

But make sure they do NOT:

Have multiple enclosed boundaries within a single Plan Region:

Have 'incursions':