I have flown the same route ATL-MIA-ATL 20 times in the last 6 months. I get a comp upgrade more than 50% of the time. For every flight I get the email prompting me to pay for an upgrade, usually around 2 weeks before the flight., immediately on getting the email, I check on Seat Alerts and there's usually 10-15 available seats in first on an A321. But I never have the ability to purchase on the App, it doesn't show up, I check back a few days later and the available seats haven't changed, so it's not that others are taking the offer..
As I said I get the comp upgrade a lot of the time, but if they are offering the upgrade at a reasonable price I don't mind paying, I just don't have the ability in the app.