I've previously asked the question and been unable to ascertain whether Frecuenta has the same award inventory on UA flights that Mileage Plus has.

But the best I can do in a few moments online is take a look at award inventory on UA.

There are certainly available award seats AUS-ORD in December. Perhaps not on the date you're trying to book? What are the specific dates involved?

Another possibility is that you're booking a business class award and the MX agent is looking for business class award inventory instead of F award inventory.

Another possibility is that on the date in question there's coach award inventory only and you're not being offered that?

Finally, AUS-DEN-ORD is a legal UA routing. Not sure whether MX will approve it, but you might try to look for award inventory that way instead of just looking for AUS-ORD non-stop.

Good luck!
Gary

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View from the Wing: A blog about Free Miles and Free Markets