These are your large analogue TV recordings I assume? Maybe the conversion routine is having a little trouble recognising them, even though they do play.
Also, try a longer recording, say at least five minutes. I don't know if that will help, but I have read some people having trouble with short recordings, usually much shorter recordings though.
I have never been happy with the conversion utility in MC, but as the video portion of MC has been rewritten by Hendrik, I believe that this will be improved in the near future. I just hope multi-channel audio is retained when this is done. In fact I don't really want a conversion utility as much as I want a remux utility, so that the original video and audio formats are retained, and just moved to a new container. For recorded TV in JTV format, that would be brilliant, even if MC just remuxed them to TS format, which is then very convertible with many applications.