Got all the way through to the 3 check marks for my LAX-LHR last week, so left it at that screen. Strolled up to the check in counter upon arrival at LAX. The agent knew exactly what the App was, but needed to see a screen with a check mark and a picture. News to me! Dug out my paperwork instead and whilst the agent checked me in and checked my docs, I played with the App and noticed that I had to go way way back to the main screen and then select a different option to get to the required all screen!
Next time!