The problem with Weeds in the show's later seasons is superficially one of setting. Agrestic, the stereotypical suburban town that was home to the Botwin family, was so central to the show's premise that the problem was also ultimately a character issue as well.
Weeds was first presented as a tale of a suburban soccer mom forced to sell drugs to support her family after the death of her husband. After a few seasons of the show, the family was forced out of the suburb and it seemed that Nancy had made enough money that she could have gotten out of the game. At this point, the show ceased to be a commentary on suburban survival and became about what possessed Nancy to move from selling drugs out of necessity to being in the game because she wanted to.
Weeds never came up with a good answer, though they had five more long seasons to figure it out.