After much investigation I discovered the obvious... (I truly believe Occam's Razor applies to almost everything in Revit.)
Basically, your "Bottom" and "View Depth" cannot be lower than, or on the same level as, your underlay. I will refer you to this fantastic image again:
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!
Sounds confusing but the basic rule is simple: Your current views depth cannot be below or exactly on your underlay level.