Seems to happen at random. We've got 25 Mailbox DBs. Generally, Netbackup reports that all have been backed up successfully.
BUT, sometimes Exchange 2013 reports these as backed up (and clears down the transaction logs) and sometimes it does not. Some nights some DBs work and some don't.
My understanding is that Netbackup somehow tells Exchange its completed and it should flag as such and clear down the logs. Where to start troubleshooting this one?