Edit: The cube and stairs were not marked as static. Problem solved.
I have some stairs and a platform, which is a cube, but the navmesh isn’t appearing on them. The stairs have a mesh collider and the cube has a box collider. How can I fix this?
Are you sure your model is in correct scale? Grid makes me think it is, but check if the unit cube fits there OK.
Next I would check the Max Slope, it might be that the angle is too steep to travel up. Also if the Step Height is too low, your steps will act like walls.
And you can try adjust Min Region Area under Advanced section. if it’s too large value, it might cull off that platform top.
EDIT: Noticed you had solved your problem 
