iMovies will not import into iBooks Author

I am trying to create a life history which has a lot of photos and short 6-8 min movies created with iMovie. Everything went well at first but now when I try to import a movie it won't work because it says it is optimizing the movie and does not go any further. My first few movies imported without a problem. It seems there is some sort of limitation on either the size of the imported video or the size of the iBook. Any thoughts or suggestions?