If I enter in search field eg. 'vlc' Discover finds nothing. If I write in terminal apt-get install vlc I can install the application. Plasma 5.7.5, Kubuntu 16.10, Qt ver. 5.6.1, kernel 4.8.0-26
This is a bug in Ubuntu's appstream. Please report to Ubuntu. This can easily be tested by "appstreamcli search vlc"
Ok, I reported to Ubuntu.