segment by segment is the only way
I tried YYZ-FCO via PHL.
I made the mistake of telling the agent YYZ-FCO - there was no going back, only a few options showed up on her screen, and YYZ-PHL-FCO was not one of them. There was no convincing to go back and check it seperately
HUACA - same voice HUACA same voice HUACA - different agent - and voila. Told the man to do YYZ-PHL, then PHL-FCO, of course it was available