Character (with A.I.) not able to navigate through stairs

Unfortunately I got no access (at the moment) to the meshes of the project.
Anyway the “Agent Max Step Height” in the Navigation Mesh settings helped me a little bit, but unfortunately seems impossible to cover all the area of the project.

Just in case I’ll get the access to the meshes, could you tell me please in what I’ve to take a look to try to fix the problem? Normal problems of geometry maybe?

ps = seems that the navmesh now it’s entirely broken and doesn’t work anymore after I tweaked those settings. -.-"
Of course I reverted all my changes to default, but it’s still broken. A message appears at the top-left corner saying:“NAVMESH NEEDS TO BE REBUILT.” …darn!