I too had similar problem last weekend. I found my issue is called the "Tivo Time warp" issue. This is where the Tivo thinks the time is incorrect and stops indexing the slice data.

Here are some links on how to fix it:
http://minnie.tuhs.org/twiki/bin/vie...msAndSolutions
http://minnie.tuhs.org/twiki/bin/vie...aveNoGuideData