I'm running the latest iMovie purchased via the App Store. I've imported videos in the past from my gopro but I'm having a problem with an event. I have several vids to import as an event but two problems. The thumbnails take 3 hours to generate but then they're all grey anyway. Also about 50% of the videos I can make selections from. If I drag the into my project I get audio only. It i try to preview them in the event library it skips back to the first clip in the event. I've deleted my thumbnails and restarted twice. It rebuilds thumbnails but no change. Any ideas what the fix may be?