sandraEDX:
Welcome to the forum.
Here are two options to check:
1. The Walls are Trimmed by a hidden Roof: turn on all layers to see if there are hidden SEO Operators.
2. The Walls are Cropped (old command before SEOs were possible) rather than Trimmed. Select the Wall, right-click to get the Context Menu, then see if the command "Undo All Crops" is there (near the bottom of the menu).
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David