Portions of Imovie video are blackscreen

I've made up a nearly 5 minute video in imovie 10. Running Catalina on a Macbook pro 2012. I've done other movies but never had this happen

The movie was working fine. Then I tweaked some parts near the end.

Now, nearly 2 minutes into it, the video portion goes black, and stays black to the end (with the exception of white text that appears). Also, the audio still plays.

When I exported the movie, it is a 1:51 movie, not the nearly 5 minute long movie. So, it has full video but ends prematurely at 1:51.

It is as though, somehow, I've selected only the first 1:51 to be saved/exported. But I didn't try to do that. I reselected al the clips, just in case. but still have the problem. In the screenshots below, the first is before 1:51, when everything is fine. The second is after 1:51. where the video is black (but as I said, does show text and plays audio).

[img]https://i.imgur.com/Bz7UpZX.png[/img]

[img]https://i.imgur.com/B9WqFf3.png[/img]