I am having huge problems with my iMovie. At first, I noticed that when I glided my cursor over the top section of my project (the edited bit) the sound or video would not play and show up on the preview screen. The whole top section was unresponsive whenever I tried to interact with it, I couldn't cut or delete anything. I also couldn't export what I had done. So I clicked on the bit where it takes you back to review all my projects, and then I was stuck there. It won't let me go back onto editing the projects, the option is greyed out and double clicking doesn't work.
In my iMovie 11, I have two projects and 11 hours of footage in one event. Understandably, this could be making my computer slow and unstable, but I think the real start of the problem occurred when I tried to work on it on another account (there are two accounts on my iMac). I copied my project and the folder which I had been using for everything I had put into my project (video, images, sound files). I dragged the copy to the Shared folder and I opened iMovie on the other account and imported everything in that folder and opened my projects. It didn't work, I think it either froze or nothing happened at all. When I went back onto the main account, this is when the problem started, as I explained in the top paragraph. I have heard that it must be avoided, having two accounts logged on, while using iMovie.