Preview pane in iMovie

Trying to figure out why the preview pane in my copy of iMovie (on my Mac) is not working.

When I import media into the Project Media pane, I can see them there. If I select them, I can hear audio, but the preview pane remains black (same thing in the initial "Import" window - I can hear audio, but the preview window is black). Once media has been imported, I can drag it into the lower window and edit or manipulate it. Everything appears to work except the preview pane.

I've tried recording videos on my iPhone and in QuickTime. I've imported various pictures (jpg / png). Same thing. I can see them in the Project Media pane and add them to the video in the lower. pane It doesn't seem to make any difference what media I use, none of it shows up in the preview pane.

That's not the case with iMovie on my iPhone and iPad, I can see the previews on both of them, but not on my MacBook.

What am I doing wrong? What setting have I missed?

Thanks

J.