If email is delivered to a PST when Outlook is open then the email will not go to a BB. Wireless email only works when in the Inbox in th Mailbox, not a PST.
That's a strange setup.
You would need to know hen the user received an email that did not reach a BB and look i the MAGT log around that time for that users email address.
This sounds environment/user related seeing as the BES works most of the time except for some users and it's intermittant.