I've read about similar problems, but I never saw a satisfactory solution, so here goes... I have a 3 part video from YouTube that I got with DownloadHelper, in mp4 format. I want to stitch the three pieces together in iMovie, so I have one video instead of 3. The first part seems to import normally. It copies the file over, and spends some time generating thumbnails. Then it duly places the video in an event. The other two parts take maybe 5 seconds to import. No thumbnails, no nothing. All three files are in the iMovie folder, but the last two refuse to appear in iMovie itself. Many thanks for any ideas here. PS — The video codec is the same as other clips that I imported successfully some time ago: AAC, H.264.