Slanting rafters

Hello, why in some models I can insert carrying slanting raters (under really rafters) or on other I cant?

Ok, in picture is shown mine variant of slanting rafters standing under the really rafters, but in attach model I cant insert them why? Theses files (model and picture is similar projects: one is with 4 columns, second - with 2 columns and other 2 corners is standing on brick wall). Here is second variant with correct slanting rafters...