Navmesh not drawing inside building

I have made a simple tower model in Blender, set up the collision and navmesh and am attempting to get AI to walk from one side of the upper platform to the other.

However, as you can see, the navmesh fails to generate inside the building:

The inside lower floor is successfully generated, as is the top. However, this “inside second floor” seen above is not being generated as pathable.

Any thoughts on how to fix this?