Originally Posted by Steve GadFly

The exact same thing happened to me years ago on vacation to POP. No problem with EUA and companion upgrade on the way down (in fact, we were the only people in F on the flight). On the way home, I was told I couldn't have a companion upgrade because of a "security issue". When I asked for a supervisor, I was told it was a "weight distribution issue". I wound up sitting next to an empty F seat for the flight back to EWR. A call to WeCare got an apology & admission of wrongdoing on the part of the ground crew in POP and upgrades for Ms. GadFly and myself for an upcoming trip.

Good luck.