Originally Posted by BigMoneyGrip

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

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.