Backup with errors

My backup runs every day, but for the last few weeks it says it has completed backup with an error that 1 file was not backed up.  It tells me to check the log for details.   The log does not tell me anything about which file is not being backed up.  There are over 16,000 files being backed up each day so I can't check each one to see if it was backed up.  How am I supposed to know what file is being rejected each time?