FFmpeg file convert problem

[The Little Guy]

Why does my FFMPEG successfully convert some files to a flv format, but not others? it will convert some mpg file, but not others. Why?

 

Does it have to do with my execution code?

<?php
// The following file names are examples:
// $dir = directory name
// $fileName = myfile.mpg (or some other video file name/extension)
// $flvName = myfile.flv
exec ("/usr/bin/ffmpeg -i '../../tzfiles.com/users/$dir/$fileName' -sameq -qmax 100 -s 320x240 -aspect 4:3 '../../tzfiles.com/users/$dir/$flvName'");
?>

 

Could it be that it doesn't have the codecs installed that the mpg files require?

 

What happens, is when I upload the file, it will run the file successfully, Which it uploads the original mpg file, then converts it to a flv file, then deltes the mpg file. It does all of that, but the flv file is 0 bytes long. Any reason why this could happen?