My wife and I have purchased award flights for one of our sons (who lives in St Louis) using UA miles. I have seen a lot of these married segment issues on award bookings when I try to book him to/from STL via ORD for other destinations. .

I have had good luck booking award flights between STL and Europe (and other destinations) through other *A gateways (specifically EWR/IAH for UA and YYZ for AC). I would try those as well.

IMO the 800 pound gorilla in the room for this discussion is booking award flights immediately around the Christmas/New Years holidays. I have done this a bit myself and have often run into difficulties as these are a bad combination of high demand and limited availability (at least for awards). Not saying it will be impossible, just that one of the best ways to get routings you like in awards is the ability to be flexible with your dates.