This happened to me regularly when I was US based. The SEA and SFO BA lounges often turn away BA members flying AA (and now AS) on domestics. Every time it happened I threw a complaint at BA and they gifted 10-15k avios as an apology. I ended up just switching to Priority Pass / Amex Plat lounges, but this was a few years ago when there weren't 20-30 person lines to enter these.