if you have set their preference to "builder" that is all you can do. they have a mind of their own. if they were previously researchers and have high research skills they seem to get tired of building and revert back to research every now and then.

some have suggested at night leaving your game on half speed and make sure the villagers are doing what you want them to be doing and then quit the game. when you come back in the morning there will be a good chance they will have continued whatever they were doing before you quit.

Good Luck.