Enemy stops when player on higher place

Hello all.
I am making a simple 3D shooter as tutorial project. I create an enemy, and add to it a NavMeshAgent and standart AI Character Control script, then set player as target to AI Character Control. While enemy is alone, all works fine, it chase player as I want, if I jump on higher place, it starts to run around that place.
Then I add another enemy (a copy of first). And here is a problem. While I’m on same surface with enemies, all works fine, they chase me. But when I jump on a higher place (on a box, for example), enemies stop moving.
Why they stops and how to fix this?

Enemies move along the NavMesh, if you jump on box (where is no NavMesh), enemy can go in wrong direction. Try to find how to add box to a nav mesh.

Please check this tutorial: Offmesh Links - Unity Official Tutorials - YouTube