Sometimes the brick wall is created when an ancestor's mother marries after the death of her husband. The problem is that if you do not know the names of the parents, it can be difficult to locate a marriage record. If you do know the names and a family disappears, consider the possibility that the father died and the mother remarried and the family is "hidden" under this new last name--whether or not the father adopted the children officially. Many didn't.
I have several families in my own research where the remarriage of the mother complicated the research. Some will be featured in upcoming columns of "Casefile Clues."