It may be a licensing issue. The last paragraph in the link above states:
If you can't access a song in iMovie
“If your Music library includes protected files, they are unavailable and do not appear in the browser.
To use a song you’ve purchased from the iTunes Store in your project, you have to be the copyright holder of the song or have express permission from the copyright holder. For more information about using content in iMovie see the iMovie software license agreement by choosing iMovie > About iMovie, then click License Agreement.”