My downloaded content wont load

When I try to download something I bought from the marketplace, like Advanced Dragons 2 or Mutant Caves, it doesn't load. It does respond to when I want it to download but it doesn't change or make any progress at all, one time I waited around 10 minutes and nothing changed, it just stays at 0% done. And if I delete Minecraft and reinstall it, it would work normally but just for a few downloads or only for a certain amount of time. And I don't want to lose my worlds.