Customizing models of houses I find in 3D Warehouse problems

I enjoy customizing models of houses I find in 3D Warehouse. Often, though, when I try to “omit” a portion of a wall by selecting it and drawing straight lines to enclose the area I want to delete, the wall still remains one piece. I am careful to make sure the lines connect with each other and to the bounding edge(s) of the wall, but it makes no difference. I must be doing something wrong. How can I get it right?

Perhaps it’s a component? If it is, double click on it, and see if you put it in edit mode.

I’d like to know the answer too. I have run into this many times on my own geometry. I have house models which consist of a single group (no components). Most of the time when I edit these and click on a surface, it will be shaded to the borders to indicate selection. If I add a line dividing that surface, I will be able to click in each ‘piece’ to see that it has really been divided into separate entities. Occasionally I get a surface that will not divide. Most of the time this is a surface that is not on X,YorZ planes, but at an angle.

Can you upload an example that shows this problem? That will make it possible to determine whether it is a bug, a flaw in the model, or a matter of technique.