Nav Mesh doesn't detect doorframe

Hi, I've got a little (or maybe bigger) problem with nav mesh. It detects the floor and the walls great but if there is a "hole" in the wall, it doesn't add a path through it. I also tried to change the radius to 0 and it still didn't work, so there must be another problem. An image of the problem:

My wall has 3 sub-objects. Each one contains a box collider to make this doorframe walkable (works well with Char Controller). I can't use Mesh Colliders because of some bugs with small objects which fell through, so please don't suggest to add mesh colliders :)

I hope someone has a solution for this. Would be sooo awesome!

Thanks in advance!