If, as Joine The Great says, you have the chief teach the children regularly, you shouldn't find this problem. I've seen it several times when I've left the game for several days, so that a new generation of kids has grown up without any teaching.

A similar situation can also appear in VV4. Although most villagers have gained skills in the nursery, they tend to hang about without starting work of their own accord. Once they are put to work on one of the skills they've learned, though, they will normally carry on without any further interference from the hand.