I noticed my Loved tracks playlist was far behind my recent "lovings", so I checked plugin, which was fine and "Sychronise loved tracks" was active.
I created a new Loved tracks playlist, and all the missing tracks were added.
So I tried to love a track, close MB, check Last.fm to see it was actually loved, then start MB again, but the track was not synchronized.
I created again a new Loved tracks playlist, and the new missing loved track was added.
So the plugin adds new loved tracks only when creating a new playlist, but does not synchronize the existing one.