ffmpeg wmv to flv?

[eluser]newbie boy[/eluser]
i have this code...

Code:

exec('ffmpeg -i '.$uploadfile.' -f flv -s 320x240 '.$new_flv.'');

which converts video file to flv file...

works perfectly with mpg file...

but not with wmv file coverts it to flv but with the output of 0kb...

need your help with this...

thanks guys...