Navmesh on self-created terrain not working?

Hi Guys! so, I wanna be short on this one. I`ve exported a map from minecraft into blender and from there as an fbx into Unreal Engine 4. everything works fine, even the collisions, but, when I put a Navmeshbound Volume on the Terrain it just doesnt work. But it does work on the standard boxes you can drag from the “Baisc” panel into the world. What should I do guys?