6 mailbox servers, one DAG. The status of most of the database for content index is healthy except for few mailboxes on 2 mailbox servers. Tried some of the below mentioned troubleshooting steps.
restarted both the search service
restarted the server
Update-mailboxdatabasecopy -identity "DB11\mb01" -catalogonly
Deleted the index catalog folder to rebuild the content index
"ContentSubmitters" group already exist in Active Directory.
Still No luck with having the issue fixed. looking forward to a solution.