That's one of the weird quirks in CS planes. in order to get a TD you need to not only select a Star, you have to select a transition. That's fine when there is a trans to select from, but sometimes there's not. In that case every waypoint up to the last will show cruising altitude, which of course is wrong. You can fix this by setting an End of descent point in the legs page, or what I do is set the last waypoint to the minimum altitude before approach on the legs page. then the FMC will calculate a descent profile to that point and I'll get a T/D.