Okay, I've found where the problem is occurring, but still no idea why. See the attached images. I copied the first floor into a new file, and it was still not rendering the upper roof. I started deleting wall bits piecemeal, and lo and behold: it's something to do with the upper right protrusion. When I delete it, the roof generates as it should.
Is there any way to figure out why this is happening? Again, I have an earlier version of this floorplan where that part of the house is identical, yet it did not have a problem with the roof autobuild.
Thanks!