Some videos are encoded using codec's that are not supported by the base Windows operating system. Or perhaps you've installed a player or media editor that has over-written some of your codecs.
When you try and convert these files using Replay Media Catcher you may receieve errors such as:
Converting - [flv @ 00AA6C70]Unsupported Video codec (7)
To resolve these issues please install a codec pack such as K-Lite.