What would cause some vids to not play?

I've noticed this before, I've had videos on my computer that play locally just fine, but when I uploaded them to the vshare script, they wouldn't play. If I go in and edit the video, resave it and then upload it again, it plays.

Same thing happened to a friend of mine. She had a video that she made, she was able to play it on her computer but when she uploaded it, it wouldn't play via vshare site.

Any ideas? Codec or something?

yes most likely codecs, also most people are running old versions of ffmpeg/mencoder....the newer versions have better support for other codecs.