The licensing process for some songs is not yet complete. Although these songs are not available for immediate download, they can be purchased online (either by credit card or on account) and will be emailed to you within 24 hours Monday-Friday (usually the same day). Songs ordered Saturday and Sunday will be emailed on Monday at the latest!
These songs show up in your library with a "Via Email" notation. Once the licensing for the song has been obtained, these songs will automatically be available for download.