Hi sunnyshines,

You may have been trying to load non Video-SL compatible video files, possibly corrupt. Have you analyzed the video files that you have in your library ? This will show if any files are corrupt.
You can find a list of supported video containers and codecs here: sera.to

And a description of how they work here: sera.to

If you are using iTunes videos they may be protected. iTunes music store video's are often protected so they will only play in iTunes. If you are purchasing video files from the iTunes music store choose files in the iTunes plus format, these files are not DRM protected and will play in any program.

Hope this helps if you have any further questions let me know.