Has anyone else not been able to check into fights 24 hours in advance with either the iPhone app or United.com? It has happened to me on two domestic fights and two international flights in the past six weeks. The error message tells me to see and agent at the airport. When I call the United Premier desk they can't explain it to me!

Is there a chance the TSA has flagged me? I do get TSA pre-check everytime so that is not a problem.

This has not happened on EVERY flight in the past six weeks, but it has happened over 50% of the time. It all started after I traveled to Mexico City for a 3-day weekend by myself. I have noticed in the past if I take 3-day trips internationally as a solo traveler Passport Control asks much tougher questions on return. Am I paranoid or could TSA think I'm up to something?

If I don't check in 24 hours in advance, I'm lower on the upgrade list, correct?

Thanks!