When you work with jwplayer you need to make sure you-
- Check the supporting file documentation http://www.longtailvideo.com/support/jw-player/jw-player-for-flash-v5/12539/supported-video-and-audio-formats
- If you are using anything other than mp4 its fine.
- If you are tying to play mp4 files and you are having problem with it (video not playing before it fully loaded) you must swap the index of the mp4 files using this app http://renaun.com/blog/code/qtindexswapper/
I think that all you will be need to start with jwplayer.