Why am I unable to fast forward movie files?
This issue is caused when the movie file is corrupted or protected. For example, if a movie file is downloaded improperly or was not completely downloaded it may be corrupt and prevent you from fast forwarding the video. If you're sure this video is not protected, try downloading the video file again.
There are also numerous programs available to correct many different video corruption issues.
Some videos, especially online videos have protection that prevents the file from being fast forward, which helps prevent the user from fast forwarding commercials. Bypassing these protection schemes is often against the rights you accept by watching the video.