You might be dealing with married segment logic. Try booking as multi-city. I had the same situation trying to use a MP Visa companion cert - ITA showed T availability but UA booked in W for my SMF-DEN-CMH outbound. When I booked it in multi-city (SMF-DEN, then DEN-CMH) I was able to get T to price out on ual.com. I put the itinerary for two passengers on hold, then called and got them to apply the Zone fare, saving myself about $90. I then mailed in the cert and final ticketing occurred a few days later.