AV Cables not working (Toshiba SD-H400)?

Has anyone run into problems using their AV cables with their Tivo? I'm trying to set up my digital cable box (it is one of the small new ones from comcast that people keep having issues with) and was using my AV cables (this was suggested in a thread on this board as a way of getting the box to work). Well, nothing worked. I finally backtracked and tried different ways of hooking things up. My suspicion is that my Tivo just isn't sensing/reading/whatever the right word is the AV cables period. My Tivo works fine if I hook the cable (without the box) directly to the Tivo and then use RF cables, but I get nothing if I try to use AV cables.

I have a Toshiba SD-H400 (a single tuner with the dvd player but not the dvd recorder).