ExpertFlyer is having issues right now, but there is probably no married segment inventory on flight pair. There has to be married inventory on routing to sell a through fare on routing (not just inventory on individual segments). Domestic FC fares actually prohibit routings that involve D1 flights, but they are looser on international FC fares. However, it's not unusual to zero out married segment inventory on such flight pairs which will prohibit routing from faring out. You need to buy segments on separate fares in these cases (DL won't show broken fare options when routing is valid for covering through fares and there is no married segment inventory on route).