(Akregator version 5.0.2, but this isn't currently selectable in the bug tracker) The default shortcut for Next Unread Article is "+,=". Obviously this is meant to mean "+" or "=", but what it actually means is the user has to press "+" and then "=" to advance to the next unread article. Workaround is to define a custom shortcut.
I fixed bug in 5.0.3