If you have problems placing rooms in Revit the issue may not be with your Walls, it could be hidden in the Computational Height. By adjusting that parameter (perhaps to 0′) your rooms may suddenly “find” walls they had previously skipped over for no obvious reason . For more on computational height click this link:

http://docs.autodesk.com/REVIT/2011/ENU/filesUsersGuide/WS1a9193826455f5ff-54f556ca1166ac9ee9e-4ad8.htm