According to this KB article KB2807668, Exchange 2013 database content index fails due to missing AD Group named ContextSubmitters. The KB says,

This issue may occur if the search platform tries to check its membership in a security group that is named “ContentSubmitters.” This group is not created by the search platform or by Exchange Server 2013 and is therefore not usually present. Although the check usually fails silently, without any consequences, an exception sometimes occurs. This causes the search component to fail.

The solution from the KB is

To resolve the issue, do the following:

1. Create a new Active Directory group that is named “ContentSubmitters” and then grant Admistrators and NetworkService full access to the group. This is a dummy group and should be used as a placeholder only. You might want to add a description so that the group is not removed.
2. Force or wait for Active Directory replication.
3. Restart the following services:
   • Microsoft Exchange Search
   • Microsoft Exchange Search Host Controller

Now if it is not the issue, you can fix the content index status by updating database copy, using following command on only on failed databases on Exchange Shell.

Get-MailboxDatabaseCopyStatus * | where {$_.ContentIndexState -eq "Failed"} | Update-MailboxDatabaseCopy -CatalogOnly

Enjoy!