Knife tool just not working on simple object I'm trying to slice my 'house' so I can create a ridge, but the knife tool doesn't seem to work on this object at all. Despite snapping to the verticies correctly, as soon as I press enter, it just disappears instead of confirming the new edge as normal.
What am I doing wrong?   My Blend is here:
Not sure why, but if you uncheck the 'Occlude Geometry' option it works correctly.

It may be because the top face of your model is not an ngon (has 5 vertices).