Anyone had this experience...?

I have a trip with a BA flight and then an AA connection, all on the same PNR. I used the iPhone app to check in and got the mobile boarding pass on the outbound from LHR, and since I was travelling with hand baggage only I would have liked to have got my AA BP at Heathrow to save waiting in line at Miami. Unfortunately the agent at LHR couldn't get my onward AA BP printed "because you checked in online". This is not the first time this has happened on this kind of itinerary, and the same thing happened today when I checked in for the return flight to MIA - I used the AA site to OLCI yesterday and lo and behold the check in staff who issued my BP to MIA couldn't give me the one for the MIA-LHR sector.

Does this happen to anyone else? It's incredibly frustrating, and for 2 airlines in the same alliance it should be easy enough to issue BPs!