From my searching, this is a previously discovered issue... but it's been around for over a year, as seen here:
The reason I bring it back up is because 1) That bug report was marked as "Resolved". 2) It wasn't fixed.
I have been dealing with the same issue today, where Hearthlings will refuse to place easily reachable items within a building (specifically ones that were placed as part of a schematic). I thought it might be my specific building design, but when I used my schematic again, it was inconsistent whether or not they'd complete it. As one person mentioned in the thread I linked, saving and reloading fixed the issue, but obviously that isn't an actual fix. I'm guessing the game isn't correctly checking for incomplete tasks, and finds the incomplete tasks when you reload the save.
Steps to reproduce:
Tell your hearthlings to construct a building.
Hearthings to finish placing items (doors, chairs, beds, lights, etc).
Hearthlings would become idle without placing the items.
Versions and Mods:
OS: Windows 10
GPU: GTX 1070