I had an interesting bug when constructing a church.
The builders would never finish it, despite having both builders and materials available.
To solve it, I had to:
- Go in building mode
- Delete the parts that weren't fully constructed
- Accept
- Go back to building mode
- Place back the parts.
I think it's linked to trying to modify the building after it's been started.
Possible steps to reproduce:
- Build church
- Half way through, notice you've made a mistake and edit a part.
- Accept.
That's what I did anyway, and only those parts were affected, so I assume it's the issue here.