I would like to reference this post, which was marked as solved, but wasn't really solved since the customer simply found a way to work around the issue. I would also like to point out that my organization is experiencing the exact same thing: https://www.symantec.com/connect/forums/clients-have-stopped-auto-upgrading-1217-14#comment-11767431
Test upgrades went fine, as did the initial auto-upgrade process for most clients. Around 60% of the clients auto-upgraded from 12.1.7004.6500 to 14.0.1904.0000, then suddenly, the auto-upgrades stopped. We have no bandwidth limitations in place, no throttling, etc. All of these clients are on the internal network, and all are Windows 7, 8.1, or Windows 10. For redundancy, and because of network routing issues, we have a separate SEPM server installed in the DMZ, only to manage Windows servers. These also stopped auto-upgrading. Same SEP client versions (old and new).
I've tried forcing a policy refresh from a few of the clients, made sure the last check-in time is the current time and checked the application logs, and the install isn't even attmepted on the client.
I enabled Sylink logging, moved a problem client to a new upgrade group with the same upgrade package assigned that was working before and have the logs available. Can someone let me know what I should be looking for?