X1 Missing walls

[orbitalsanders]

My first attempt at generating walls on a second floor 12 pitch hip roof, has produced an unexpected result.

I created a second floor then moved two walls in and changed them to interior knee walls. The other end walls are full hip as well as the first floor end walls.

When I used the Build Roof command no wall appears between the knee wall and the ceiling.

Any ideas are indeed welcomed.

Thanks