Doubleclick on the movie and go to tab Files. Here you can add/remove files. If there are too much movies to process then go to the device scanner and read the folders again. This will add additional parts of a movie. The last possibility could be to go to Tools -> Collection Maintanance -> Re-read codecs. But the last option I havn't tested therefor I'm not sure if it would do the job.
In order to achieve what is possible, you have to try the impossible over and over again. Hermann Hesse