I've been posting about it all week, but I had problems because the video was 720x512 that the client gave me that another company made for them, and on top of that it had black space at the top, and all needed to be made broadcast safe.
Anyways, all I can say is to check and make sure that the format you're rendering to specifically supports closed captions, because I've seen that for the same video, certain formats (and their different resolution requirements, more accurately) don't.
http://www.scenethroughglass.com