Was in trouble with Copa interlining my bag on separate tickets. Got a business class ticket with Copa BOG-PTY-FLL and separate ticket with United FLL-IAD with a very short connecting time in FLL (no chances to recheck the bag, just a time to pass the Immigration and to run to my gate).
United told me - just show your second ticket and your bag will be checked until the final destination, we got an interline agreement.
Thanks God - was very early on a check-in in Bogota. They vigorously denied to check the bag to IAD and told it is a policy if they don't see my next flights in their system and tickets are separate. We argued for more than an hour! Spoken with the check-in supervisor, called United 1K, and they talked to Copa... As a result, they did me a favor and called their high-ranked manager in the city office and made an exception. But I was very close to loose my flight from FLL (the last one on that day) because of the situation.