Package not found when using Github NPM package registry

I had a project which works fine when I use npm package registry or nexus. However, when I switch my registry to and run npm install, it returns a E404 not found for many of the packages. I have to install these packages from a different registry. From my understanding, GitHub registry should proxy to npm for packages that are not on GitHub. For example, one of the error messages when I run npm install is: 

npm ERR! code E404
npm ERR! 404 Not Found - GET
npm ERR! 404
npm ERR! 404 ‘classnames@^2.2.6’ is not in the npm registry.
npm ERR! 404 You should bug the author to publish it (or use the name yourself!)
npm ERR! 404 It was specified as a dependency of ‘playground’
npm ERR! 404
npm ERR! 404 Note that you can also install from a
npm ERR! 404 tarball, folder, http url, or git url.