Have you tried putting someone in front of the first hut your builder built, sometimes this will improve their construction, I've found that if you want to raise the building bar doing this helps. Sometimes it's says they're fixing the hut and you hear a hammer banging away but then they shake their head, so like most puzzles it can take a few tries. I've even set someone to work on it so many times to improve their building it eventually says the hut is in good repair.