Some of them are just plain stubborn! So you just need to keep at them until they get some skill in the trade you want before they will do it on their own. Once they get to adept, they should keep at it, but of course if they have more skill in another trade, they will prefer to do that over others, but they will stay on tract as long as you have the skill checked that you want them to focus on. If they have no skill at building and you have building check, they will not build until you train them.
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The VI