I have been working with building a lot, and I realized that the reason buildings won’t complete relates to scaffolding/ladder issues. The game is unable to place a secondary ladder to reach a needed ladder. This keeps the building from going up. If the scaffolding requires lower scaffolding to reach (which was part of other construction), sometimes the lower scaffolding gets removed first, and the second one can’t be removed, and then all of the remaining scaffolding may refuse to come down.
You can build your own ladders to make the building complete, assuming you can find the piece that isn’t reachable.
In real life scaffolding is an entire story tall and allows you to reach the entire floor height. If the system were revised to reflect real life, it would make building MUCH more enjoyable and remove some of these problems created by single unit scaffolding.