To the point. Pause the game, that’s it!
I noticed that when you open the therapist window to check their tasks, if you pause it they will start changing tasks after a few seconds. Most of the idle ones will start something useful!
When you get a good amount of hearthlings not on idle anymore, just unpause the game and watch they finally working!
I had started a second village very far from the spawn point, and although they build all the blocks in it, they were refusing to add furniture, decorations, windows, doors, etc… With this trick they did it! Although it took along time nonetheless.