iMovie and video references getting lost

[hugoniot]

I edited a video with iMovie, exported it, and then removed iPhoto event containing my video clips from the phone. A week later, I wanted to do some additional editing, so I added the iPhoto event back to my phone, checked that all the video clips made it to the phone, then went to iMovie to re-edit my work. But none of the video clips were recognized within iMovie. The still pictures still come up just fine, but just get the blank reference with the danger yellow triangle and an exclamation mark for all the video.

Any suggestions? I have an iPhone 4 running 4.2.1.

Thanks!