iMovie 10 Ducking doesn't work

I have tried several attempts to duck the sound of video and have a music track play on top of the video, however since the new update the ducking feature is less user friendly than the previous version, and despite all my attempts I still can't get this to work.

As you can see, I have music below the video, and have clicked the video and clicked 'lower volume of other clips', but does nothing to duck the sound.

NOT happy about the new iMovie10. :-/