Problem:
Although you have set a level as an Underlay, it is not visible.
For example you are in a roof plan and want to see the first floor as an underlay.
Cause:
The "Bottom" and "View Depth" cannot be lower than, or on the same level as, your underlay.
It makes sense when you think about a roof plan...
If your roof plan is set to have the First Floor as its "Bottom" and "View Depth" and you hid the roof the walls of the first floor would be seen.
Therefore, when making the First Floor the Underlay nothing shows up because it is already seen! (although hidden by things above it, like the roof).
Note the same problem can happen with an Underlay of a level above. If your "Top" is higher than the level you have an an underlay.
Solution:
Change your current view's depth so it is not below or exactly the same as your Underlay level.