Reminders are a function of your Primary .PST file, or the .OST file for your Exchange account.
If you create reminders for items (mail, appts, etc) stored in a seperate folder, you must enable that.
To fix this, in Outlook 2007, which I’m also using, follow this procedure:
1 Select the “Folder list” view – in the menu choose Go, Folder list (CTRL + 6)
2 Collapse all the main folders so you have just ‘+’ signs
3 Right Click on one of the folders that likely contains the appt.(not your primary Mailbox)
4 In the pop-up menu, choose Properties
5 In the Properties tab tick “Display reminders and tasks for this folder in the To-Do-Bar”
See this link for more background information: