In Sims 2 Pets, I can't seem to be able to use the interior foundation tool anymore for split level houses...

I have Nightlife, Open for business, Seasons, glamour life and pets. I have only recently installed pets and that's when I have noticed that the interior foundation tool doesn't work. I keep getting a message which says something like " can't place through wall". Even though there is not a wall there. How can I fix this?