Originally Posted by
ORDnHKG
Ok, that is the reason ! My searches require no overnight (depart PDX 11:50pm), but yours do.

Thanks anyway.
I don't think you can transit IAH both directions without an overnight or a redeye. If you get something on ITA but CO/UA refuse to book it as a roundtrip try:
1. Two one-way itineraries on CO/UA
2. Roundtrip on Orbitz