The UA website is strange in what it chooses to show sometimes and other times not.

The partner explanation above is one possible explanation that is more legitimate - UA will generally only show *A partner flights in a revenue search if it is codeshared (partner flights, as an award, have to be booked on operator flight information). If not, it will usually not show at all. One way around this is to search for the itinerary as an award, then before you redeem it at some point, there is usually an option to purchase the itinerary in $ and you should be able to complete the booking anyway. Disclaimer is that depending on the partner, the price may be full-fare or close to it (but other times available for typical discount pricing).

If it is all UA metal (or even if not) and it is not showing, another option is to build the itinerary using he multi-segment search and entering the routing you want segment by segment. That usually (but still not always) brings up the segments. Alternatively, using a tool like Hipmunk or even Kayak to find the flights, then click on the link to book through United is another way to make this work for the revenue booking Why these engines can bring up itineraries that UAs site doesn't is another question that is beyond me, but they can.