Using the Amex portal for anything but a simple round trip is problematic since it does not show all the possible options. So when you go on there to book in order get the 5x points, you really should already know exactly what you are wanting to book. If it does not come up exactly as you want it, you will need to figure out a way to tease it out, usually by changing some parameters one by one.
Since most of my trips are complicated three and four stop itineraries, if it does not show me the flights that I have already selected, I don't bother wasting time fooling around too much and just go over and book directly with the airline.