Thanks everyone for the replies.

The incident was last year at O'Hare. I was told by both AA and BA that because DC Mike under AA code 123456 did not board the flight, the bag was flagged and could not travel. It didn’t matter that DC Mike under BA code ABCDEF did board the flight, as it was almost seen as a separate passenger. Maybe just an excuse, but was something that was said to me proactively.

I’m trying to remember the exact luggage tracking information. If I remember correctly, the bag made it all the way to the BA plane, was scanned and rejected, and then ended up back in the depths of O’Hare in AA’s possession.

In terms of getting the bag back, BA was difficult and unhelpful throughout, claiming it was AA’s fault as they failed to transfer it properly, regardless of who the final carrier was.