Ducking not working in imovie

Hi... running the lastest version of imovie. All up to date.

Ducking does not work when selecting on a video clip. i.e. I want the background music to fade and keep audio on clip up.

I have tried with several different songs, close and restarted imovie, restarted computer. Nothing works. I have no transitions or voice overs. Just a clip with a song in background.

Any help is appreciated.

Thanks.