AC.com can't book it nor can an AC agent. There are limited code shares with UA through IAH. This is a holdover from the pmCO days. I had a similar problem trying to book YYZ-IAH-TUS. UA had better flights on the outbound and AC through LAX was better on the return. However in North America you can book two one ways for the same price as the return. Try pricing a one way with AC outbound and a one way with UA on the return.