Symptom
iMovie's scene detection feature doesn't work. Scenes that should be imported separately are imported together.
Solution
Make sure your video camera's clock is set correctly, and that the camera's battery is charged.
iMovie's scene detection uses the time stamp from your camera's internal clock. If scene detection is not working, you should make sure the camera's clock has been set and is running properly. Many cameras have a small battery that powers the clock. If this battery hasn't been installed, or if its power has been depleted, your camera's clock will not run and your tapes will not have the time stamp iMovie needs to detect scene breaks. Consult the manual that came with your camera for more information.