Yeah, that was a nagging problem for a small, but not insignificant number of AT&T users for several years. However, it seemed to be mostly fixed over the past two years.
But as you see, it's not completely fixed. AT&T never acknowledged the issue in the first place, but over at the AT&T support forums, you could tell that it wasn't an isolated issue or device specific either. The best that any of us fellow customers could figure was that somehow the back-end updates servers would lose the connection between an account and the IMEI of the device. Therefore, while a customer service rep could see your phone, the update servers could not. The rep could initiate a command to update your phone (a common remedy), but the update server would never see the phone, thus the phone never receives it. Customer calls the rep and the update command is sent again, the phone never receives it again... wash, rinse and repeat.
Once in a while, something finally clicks, and the updates finally come in. This started happening more often about two years ago now and those that complained about not getting updates on that AT&T forum finally started getting them.
But not everyone.
As Mustang says however, many found that by manually loading the firmware yourself, that tends to kick something in gear so that suddenly subsequent updates finally finds your phone on AT&T's network. We aren't sure what does it exactly, or why, but it's something.
BTW, Frija can't find the firmware because the phone is branded as AT&T and AT&T phones get their firmware from AT&T's servers. Not from Samsung's servers where every other carrier and unlocked phones gets theirs. Frija only looks for the firmware from Samsung's update servers.
Luckily, AT&T branded Samsung phones can have firmware loaded outside of AT&T. It just has to be proper Samsung firmware.