I think someone asked this question before on her Community forum and TAM said that's because they are not on the right centric. If you are truly hipcentric, your butt shouldn't flatten out- if you are doing hipcentric but you are actually glute or ab or omni, then it might. Anyway, that's their explanation. I don't really know.
In my experience, TAM is not great for building back there. Your butt will probably get smaller. My butt has gotten more sculpted on the sides, lifted, and smaller. It hasn't gotten rounder or flatter, just smaller. Tracy's ideal butt is like a "tight fist" and she thinks traditional exercises give you a butt like "half a watermelon". In my opinion, "half a watermelon" is not a bad thing
Anyway, the method is good and it will tone you all over, but you might want to supplement if you want to add more roundness to your butt. I'm doing Omni and the Body Calculator and Tracy both say I'm Omni and my butt has not flattened out, but then it's quite flat to begin with and it hasn't added any roundness to it. Mainly because TA works the gluteus medius and minimus and really ignore the maximus which is what adds shape and size/bulk. I think this is true for all her centrics. Some people might experience their butt flattening out simply because of atrophy.