Did you regenerate the AI grid after placing the walls?
As far as not being able to see the walls, There's a setting on the sidebar which can hide walls or other objects if checked.
Look for the setting "Fadeable" and set it to false. Usually you want the roof fadeable and the walls below them set to false.
Though this is active if a player is close enough to the wall. So if that's not the issue maybe someone else has an idea.