What are the rules to CPU upgrades on award tickets? I’m 1K and have a United CC. When I book domestic award tickets for myself I have always been added to the upgrade list and frequently get upgraded.

I booked a flight this coming Saturday for myself and 2 friends. First only has 1 sold seat and I half expected to be upgraded by now, but when I went to check under my upcoming flights it doesn’t show “CPU Eligible” like it does on my solo award bookings. Any ideas why?