Occasionally a movie file will be quite large but when it is played the total time shown is only a few minutes. The movie will begin, and then play for a minute or two but there is much more to be seen. If you move the progress indicator, a new part of the movie will appear and play to the end of the progress bar. Moving the progress indicator to various positions will result in different parts of the movie playing but only for the minute or so left showing in total time.

Any idea what causes this and how to fix it?