Navmesh not filling areas

Could anyone give some tips on how to set up a navmesh in my architectural scene please?

I have attached an image showing the issue and the settings I have used so far. As you can see, the green navmesh does not extend all the way to the edges of my geometry. I have manually added lots of box collision gizmos so that they fit each step and landing accurately.

What else should I be looking for?