I'm trying to book an AC flight using an online website (Hipmunk, cheap tickets, etc) where the goal is to have one of the legs operated by AA. In this scenario, for the return I'd like to go direct CNF to MIA (AA flight) and then from MIA to YYZ.

This flight does not appear for any of the searches I do? Why is that? The flight to CNF gives me many options with Interline partners such as TAM. AA is also an Interline partner, but this option direct to MIA from CNF does not appear and I am forced to use an option using TAM to fly to GRU and then a Star Alliance member from GRU to YYZ or GRU -> MIA -> YYZ.

What would be the reason AA is being excluded here? Are the interline agreements route specific meaning only certain interline partners will be made available on certain routes to ensure AC doesn't canabalize their own sales?

Trying to learn...