Hi Ulla,
I suspect the new walls are either above or below the Floor Plan Cut Plane (FPCP).
In the 3d window select the problem walls, bring up the walls settings dialog box and change the floorplan display to either 'Symbolic Cut', 'Outline' or 'Overhead'. This will force them to display on plan regardless of where the FPCP intersects.
It is also worth double checking the FPCP is at the correct level, as this is stored with each plan View, and can change when you switch between them.
Hope that helps!
