Yes it's the VV3's big problem (that is not in VV1 and VV2!! )
You may try to train them in puting them at "Fixing hut" but you'll have to stay with them and re-put them at the task each time.
You may also give them a potion that makes them exercising. It will increase their building skills a little if they succeed! And you'll have to do it again and again!
That all that I know.
You may look in Forums List » The Games » Virtual Villagers 3: The Secret City » (spoiler) More Master builders needed please!! and see that you're not the only one!

The exercise potionis in the VV3 guides section: [SPOILERS] Alchemy Potion Lists