Have to spoken to your BES Admin about it? Could it be a problem with Outlook 2010? That is still a beta product as far as I know and the BB isn't guaranteed to work with Beta software you know.
I'm running outlook 2010 for months now and don't have that issue. This has to be some other type of problem, I would guess unrelated to the bes. The email has to be in your mailbox or it wouldn't be going to your bb. It seems like it's a problem between mailbox and outlook. Sure you don't have any rules moving the messages or going to junk mail?