Why when using "save for web" will only some movies be converted to m4v?

I have benn posting video to a website. The best quality seems to be when using m4v format. Through trial and error I have found that by converting the orginal movie to DV format and then placing the DV file into the Quicktime Player and then choosing the "Save For Web" option with the "computer" setting checked I will get a folder with multiple files, a jpeg, and MOV copy and a m4v copy of the movie... but only on some of my movies. When I try to convert a different movie I end up with a folder of everything but the m4v file! Why do some "Save For Web" folders created have the m4v copy and others do not?

I'm using Quicktime Player 10.0