i can't add some mp4's to my playlist

some mp4 files (video's) i am able to put into my video playlist on my ipod, but some won't work.. why?

thanks SO much for whoever that can help me with this.. thx


Edit:

i think i realized why, it seems that i don't have the right converter, can someone give me a converter that converts flv files to Ipod movie file?

thanks