Archive problems

[dan]

The virtual drive on my SBS recently became full and as a
result i have had to start archiving all mailboxes so all
old messages are saved on local machines and not on the
server. My problem is this:-

I have been performing manual archives and had no problem
when archiving inboxes. However when i try to
archive 'sent items' (most of which contain around 70Mb of
messages), the .pst folder is created in the desired
location, but none of the items are sent there.

Any ideas why?

Thanks