I was troubleshooting an issue where errors were seen in the RetainWorker log which caused concern about whether Retain was archiving all messages.  I used the following to troubleshoot the issue.

In the RetainWorker log, each archived message is delimited by 60 equal signs.  Using Notepad++ and selecting 'Find All in Current Document' I found:
47,990 attempts to archive

Next I searched for:
"Message was previously archived so skipped!"
In my case 37,464 messages were skipped

Next I searched for:
"Message archived successfully!"
10,526 were archived

37,464+10,526 =47,990

So although there were errors present in the log file, Retain successfully completed the archive job.

Note:

In this specific case, the errors matched those documented in this TID:

https://support.microfocus.com/kb/doc.php?id=7019181

Location of the Retain Worker logs:

Linux

/opt/beginfinite/retain/tomcat/logs

Windows

c:\Program Files\Beginfinite\Retain\Tomcat\logs