Junior world champs that did not make the senior worlds team for their country:
Ann Patrice McDonough (not sure about her)
Junior world champs that did not win a senior worlds medal:
Caroline Zhang (not sure if she made the world team)
It just goes to show that success at the Junior Worlds does not promise success at the senior worlds.
The flip side of this is that some junior world champions did have stellar careers at the senior level (e.g. Boitano, Yagudin, Plushenko, Kulik, Mao Asada, Yu na Kim, G&G, and to some extent Michael Weiss)