I am often surprised by the check-in staff's definition of 'cannot'. Recently, due to add-on to an RTW, I flew CP from EZE to GRU, then on AA back to MIA. My colleague and Iarrived at EZE expecting to check through (especially as AA operates this for CP at that airport.)

I was told by the clerk that I had to collect my bag in GRU, clear customs and immigration and then re-check in for the flight to MIA. Hello ? No so I responded. I was not going to risk that potential delay, and additional $36 departure tax.

So with some insistence, I got to speak to the supervisor. She intially didn't agree with me, but then changed her mind as it was clear we were not going to leave this issue.

CP got their own back on me eventually, as my bag was never off loaded in GRU, went on to Toronto and it ended up with an overnight stay in Tokyo Narita.