why can’t I change the aspect ratio on input 3 of my Panasonic flat screen plasma monitor?

When I'm on input 1 and input 2 I can toggle through the different aspect ratios on my plasma TV, but on Input 3, which is the input set to my TV, I'm stuck with "full" aspect ratio and can't change it. Does anybody know why? Or what I'm doing wrong?
The input is my HD Tivo Series 3 box running through an Onkyo receiver. So you're saying because the input is HD the Plasma may just read it that way, period? That may make sense because I sure thought I used to be able to change the aspect ratio even on Input 3. The only difference is the new Tivo box.