Navmesh won't cover entire mesh's surface

I have Meshes i created in Blender that i want the entire surface to be covered in navmesh.
But it only covers a part of it (see on screenshot)
I just can’t seem to do this i need the ai to be able to walk near the edge aswell. Is there a way to get around this?
(Ps: I don’t want that navmesh on the lava either)

Anyone?..

Navmesh seems to change depending on the size of the mesh.