As pdhenry alluded to, there are a few hidden fields of guide data TiVo uses and if some of them are missing, what we call "generic guide data", it plays it safe and treats it as first run.
News programs are big offenders for this. They'll just have generic text about "news" and a date of the first time they ever aired. But the rest of the guide data is omitted (specifically, IIRC, the IsEpsidode=true flag and EpisodeID value)
The Daily Show used to be another big offender. Their new episode had proper data, but their late night reruns didn't.
If you've got a recurring problem you might see is some variation of the 'Tupper Method' http://www.tivocommunity.com/tivo-vb...71#post5682671
, a trick for using wishlists to avoid recording bad guide data repeats of The Daily Show, would work for you