Ok it goes like this:
1. Iv'e tried archiving in outlook 2013 (an Exchange account), defining the archive to apply on anything before June 2014 - the archive folder was created but nothing moved there.
2. I've concluded that may be for some reason the "Last modified date" field for all mail was the cause & so I've added the registry key ArchiveIgnoreLastModifiedT
ime = 1 under HKEY_CURRENT_USER\Software
es (as per the suggestion here: http://www.howto-outlook.com/faq/archivenotworking.htm
3. I've run the archive again but this time I've decided to apply the archive for anything before January 2015 (it's actually a client & he's changed his mind, anyways..).
4. The new archive folder was formed & mail did move there but that mail was only from between June 2014 to January 2015.
It's like Outlook decided that everything before June 2014 was already archived in the first archive I did - which is of course not true!
Now the guy has in his Exchange mailbox mail up till June 2014 & mail after January 2015 with the gap in between moved to the archive.
It doesn't matter how many more time I run the archive to apply to anything before January 2015, all mail before June 2014 is still left in his Exchange mailbox & isn't moved to the archive :(