Give specific examples Maid. There are circumstances where Theatre View won't show Unassigned for a category.
For example, are you trying to see when the [Series] field hasn't been assigned to a recording? Are you using the "TVInfo(SeriesDisplay)" expression to group by?. That won't show an Unassigned category, unless you select the "Display All as a choice" option in the View configuration, then select "All Series" in the View, and then select "Unassigned".
Doing it in Standard View as Spike describes is the easiest way to highlight missing data. Or creating a separate View in Theatre View.