I'm trying to piece together a multi-city itinerary with the Aeroplan booking engine. I was just trying out LGA-CDG-FCO-LGA.

When I selected a United flight for the LGA-CDG leg in F, it would not let me select any leg from CDG-FCO in J. But when I selected a mixed *A trip from LGA-CDG in F, then it let me select the CDG-FCO in J on Brussels or Swiss or Lufthansa, etc.

Is there a routing rule I am breaking by trying to go all UA from LGA-CDG that I am not aware of?

thank you.