I have nearly the same problem with several addon applications for nuvolaruntime e.g.
nuvola-app-amazon-cloud-player.
The package contains a metainfo file [1] according to the fedora package guidelines [2],
but doesn' t appear in the Gnome Software Center.
[1]
http://pkgs.fedoraproject.org/cgit/rpms/nuvola-app-amazon-cloud-player.gi...
[2]
https://fedoraproject.org/wiki/Packaging:AppData#.metainfo.xml_file_creation
Any help is welcome