Hi, has anyone else noticed a recurring difficulty receiving credit for Star Alliance flights from Latin America?

In Feb I flew roundtrip BWI-CLT-GIG on US. Each of my flight segments credited to Aeroplan within the usual three days or so, except for the GIG-CLT segment. To get the proper credit, I had to mail in copies of the boarding pass/ticket receipt and then call AP about half a dozen times over the next several months to press the matter. Even though everything was in order and I had all the documentation, it still took until July (!) for the miles to post properly.

Now I've just returned from an LGA-IAD-EZE trip on United. As before, three segments went through within just three days, but my long flight from South America to North America (EZE-IAD) has not. I called the service center this morning to start the ball rolling, but I fear I'm set for another several months of struggle to get the miles I'm due. I'm particularly peeved this time because this flight segment will bump me up to Elite status, with which I'd like to try upgrading myself on a trip to LHR in three weeks.

What could be causing this type of problem? In both cases, I included my AP number at the time of the reservation. I've never had a problem like this with any other frequent flyer program, or with getting AP credit from Star partners on any intra-North American flights.