ok so i finally find an answer.
first of all, here is a post by who had same problem like me. [4.6.1] - Navmesh not generating over instanced static meshes - Asset Creation - Unreal Engine Forums
seems like there is some problem about “navigation mesh generation system”. i Guess it is possible that ‘Add instance’ in instanced static mesh component doesn’t tell Navigation Bound Volume to regenerate navigation mesh. So, you need to tell Navigation Bound Volume to recast nav mesh.
So long story short, i discovered that if you press F8 during play you can edit map like edit mode. so i use that, move my instanced static mesh component a little then that makes recasting work begin, and i have got a full navigation mesh.
there is some ways to make actor to tell nav system to recast nav mesh. but i couldn’t find any way to rebuild nav mesh only by blueprint.
So, Long story short, you need to trigger engine to rebuild navigation mesh by moving an actor or execute some functions on C++.