Originally Posted by BigMoneyGrip

https://www.flyertalk.com/forum/35523045-post677.html

Same issue that happened to me a few years back happened to my wife yesterday:
She checked into her flight via the app and received a boarding pass, yet the pass doesn't scan and the gate agent says 'you are not checked in' - despite showing them a boarding pass. She was ultimately able to get on but the gate agent was telling her to go ground side to get it sorted but then finally did it herself with 5 key presses once everyone else had boarded the flight - this was after telling my wife "you aren't getting on this plane". As my wife stood there waiting to the side she was imagining missing Christmas and frustrated beyond belief at the whole situation - after all she did check in and had the pass to prove it! She even had to scan the pass to get through TSA!

The thing in common between the 2 times is both trips had outbound and return flights within one day of each other and in both cases we checked in for the return right at the 24 hour mark upon receiving the email/alert from AA. My guess is there is some glitch in a process that closes out the first flight that invalidates the check in for the return?

Going forward I will avoid checking in soon after the 24 hour mark and always get a paper pass at the kiosk as this will 1) hopefully validate or recheck in if our app checkin was invalidated 2) prevent the gate agents from being able to blame our phone for the problem.

No idea if AA IT is aware of this glitch but something they need to get resolved!