Why doesn't my simple mesh wall "difference boolean cut" the door away? I am new in Blender. I have basically done mesh walls, and I want to cut away doors.

So I select each wall, take boolean "difference" and select the red rectable (that should represent a door)..

But something weird happens. I can see the wall has been slightly changed at the door, but no cut out. 

What I am missing?

Download link to .blend file

https://www.dropbox.com/s/apv7gx2394jtu0d/blender.blend?dl=0

I have also done a video

https://youtu.be/hIt52h4-bRs

I can upload the Blender project if needed.
BMesh solver (the one used by default by the boolean modifier) don't like when two faces are coplanar, like the lower faces of your wall and door.

You can move down your door a bit (or just move down it's lower face).

You can also change the solver used by the modifier. In Solver, instead of BMesh, select Carve.