Hi,
Thank you for posting in Symantec community.
I would be glad to answer your query.
It might be the issue with SEP 12.1.1 issue, there is a fix in SEP 12.1 RU2 notes.
Clients report to the Symantec Endpoint Protection Manager as offline, even though they are online
Fix ID: 3002170
Symptom: Clients will randomly report into the Symantec Endpoint Protection Manager as offline, even though they are actively online and available.
Solution: Updated the client USN management to properly update the client status in the Symantec Endpoint Protection Manager reports.
Reference: New fixes and features in Symantec Endpoint Protection 12.1 Release Update 2 Maintenance Pack 1
It's always recommended to have SEPM and SEP clients on the same version.
Upgrade all 12.1.1. clients to 12.1.4, It should resolve the issue.