====== Fedora 21 - BUG - Gnome Software ======

  *  Does not show available applications
  *  Does not show update notifications

This worked for me:

  sudo rm /var/lib/PackageKit/*.db
  sudo systemctl restart packagekit.service
  sudo pkcon repair

https://ask.fedoraproject.org/en/question/63489/packagekit-service-fails-to-start/#63693 https://bugzilla.redhat.com/show_bug.cgi?id=1188455#c3