My guess is that you need to go into Outlook's advanced settings and check the box "Leave Mail on Server".
From the FAQ: Some email comes to my BB and some does not - BlackBerryFAQ
Outlook > Tools > Email accounts > View or Change existing > next.
Highlight the email account > Change > More Settings.
Advanced Tab > Leave a Copy Messages on Server
Check that box.
* You will need to perform this edit for each Outlook email account you have.
If this doesn't fix it, then we need more info.