Outlook does not popup reminders

First try running the command from the Start>Run box (with Outlook closed):
Outlook /resetfolders /cleanreminders
If that does not work try turning on 'Use cached exchange mode'
To do this:
Start>Control Panel>Mail>click Email Accounts button>on the Email tab highlight the Exchange account and click the Change button. Put a checkmark in 'Use Cached Exchange Mode'. Click Next. Click Finish.
Try Outlook and see if reminders popup. I have had success with this.