Carl, Well help you work through the difficulty. It is a complex fold and I can understand why it might not be instinctive to put together. this is one of our oldest kits I've built it several times and assure you all the parts do fit.
What scale are you working in. this will help me guide you, also can you send a pic of the problem. I think I have an idea of what the problem is but i want to be sure.

Knowing the scale is a help Carl.
The roof on O is in sections where the HO one is not.

Looking at the page marked under roof support. You will see a shaded area bound by a red box. This is a guide and additional support for the exterior roof piece. Since the roof has to allow for the thickness of the walls, it is about a quarter of an inch smaller all the way round but attached to each edge of the roof is a piece of wall and a tab. The wall section that represents the back side of the exterior wall is folded forward. The tabs are folded back. once folded the roof part is aligned with the red line on the roof support. this shoulg give you a stable piece that will drop into place.

Great Carl. let me know if it helps. As mentioned its an older kit and we would likely engineer id differently now but I like to move forward.
We are here to help.