My iPod won't play all of(but will play most of) the video podcasts that I...

...have in iTunes.? I have several video podcasts in my iTunes library, all downloaded and play fine on my computer. But they don't all transfer to my iPod (though most do)...I get a message that says it "can not be played on this iPod". I've done everything I can to fix (including the "5 Rs"). Can you suggest anything?