Why can't I register a Dependency from a referenced class library?

So I have some services that I have implemented in my Android and iOS projects, and I thought I would abstract them out into separate class libraries so that I could re-use them in other projects. They compile fine, but the Dependency Service can no longer find them when I do: DependencyService.Get<IWebNavigator>();
It simply returns null. Is there something special I have to do to register my dependencies if they are in class libraries instead of the main Android/iOS projects?