I had a building once that got about halfway up before stopping on me. I was so confused at the time, as it looked to me like they could climb to everywhere, why wouldn’t they build? On a whim I built a ladder going up to one of the corners. Lo’ and behold, apparently that apparently inaccessible block stopped everything. They then finished the building and removed all scaffolding and ladders.
Lesson of the story? If at first your hearthlings don’t succeed… tell em’ they suck, build a ladder, then upload your template to this thread, along with details about where they stopped, if you could get them going again, and how.