Man, that is weird.

OK, We can fix it.

First off, the top edge of the interior north wall is on the Roof layer. No big deal, but when you drill in, the edge appears to be hidden. If you want to fix it follow this procedure:

1) Open the Ruby Console (Window>Ruby Console) , 2) Drill into the North wall and select the interior face only. No other selection. 3) Cut and paste this into the command line of the ruby console:

Sketchup.active_model.selection[0].edges.each {|e| e.layer = "North Wall" }

and that cause the top edge to show when only the Northw all is active. (Alternatively, make the Roof layer active, select the edge, right click for Entity Info, and change the layer through the dropdown list. (the ruby stuff is more fun!)

Next, even with this face's edges all on the same layer, I had difficulty assigning a material - until I double clicked the face (to select the face and all it's edges) and then I grouped it. Then, right click, choose EXPLODE, and then you will be able to assign a material.

Go figure.

Todd