I am trying to look at purchased fares from ATL to BLQ (Bologna). I can get different routings either through CDG or AMS. When I then try to use the Multi-city feature to extend the layover in CDG or AMS (but keeping it less than 24 hours so it is not a stopover) I get error messages:

No flights are operated between AMS and BLQ

or

No flights are operated between CDG and BLQ

Any ideas on why this obvious bug exists, or what can be done?