In my experience, I set an alarm for 9:45 and opened the app before moving onto the actual show page around 9:50 and refreshing over and over. I kept going despite it saying the tickets were in people's carts, and was able to get the one ticket I needed. Just because it says they're in someone's cart, does not mean it's done and over - wait until it says sold out.
"Anybody that goes to the theater, I think we’re all misfits, so we ended up on stage or in the audience.” --- Patti LuPone.