I'm no techie but I found through hours of trial and error that you must pay close attention to the names of the files that you import into editing. For example, if your camcorder names a movie...MOV001_1 and you decide to change it, avoid using characters like !, &, and (). The reason is that during the import process, the software either doesn't recognize the character or it changes it and then it can't find it when it tries to edit your movie.
This worked for me, hope it works for you too! Oh, by the way, it doesn't matter if you use the special characters in your titles for the movie clips, it is just the file name itself that it is concerned with.