On AA, there seems to be a persistent lack of training on how to issue connecting boarding passes on BA, probably exacerbated by bad SABRE programming. A few months ago the check-in agent at SFO was unable to issue a boarding pass on my LHR-AMS connection, so I asked the Admiral's club to issue it. The first person was unable to, but there happened to be a oneworld ambassador in the area, and the latter one was able to issue it after correcting some passport or emergency contact info in the PNR.

The above is easily fixable on AA's and SABRE's part, and they should do so. In the meantime, I never take a "we can't issue it" as an answer: if the person who checks me in is incapable of doing it, I ask him/her to ask a supervisor how to do it...and this exact routine worked like charm last week when I was connecting into an LA flight.