I find the app for the most part very reliable at issuing digital boarding passes in the circumstances where UA is allowed to issue digital boarding passes. There are situations where they just can't issue it for a specific passenger on a specific itinerary in a digital format (either initially or at all), through no fault of the technology or the passenger, and there's nothing anyone in reservations, 1K line or 1K web support can due to override that, That could be due to needing to check in at the airport, getting the dreaded "SSSS" requirement, or hitting some other criteria that I'm sure exists but UA can't disclose, that prevents them from being authorized to issue a digital boarding pass in that specific circumstance.