Reminders, including flags and appointment or tasks reminders, only work when the item is in one of the default folders: Inbox, Calendar, Tasks when using an older versions of Outlook.
Beginning with Outlook 2007, reminders will work if the item is stored in any folder in the mailbox or PST. See Reminders for archives and other pst files if reminders are not working in other data files or if you want to disable the feature.
In order to receive reminders from the non-default folders you will need to use one of the reminder utilities listed below.
Outlook COM add-in that enables you to receive reminders from any mail, calendar, contacts, or tasks folder in your default information store (Exchange mailbox or Personal Folders file) except the Outbox.
Public Reminder Add-In provides reminders from any email, calendar, tasks or contacts folder in Microsoft Outlook, Public ShareFolder or Microsoft Exchange Server. This allows you to set reminders on items in any folder in your profile, including mailbox, PST file, or Public Folders. Reminders can be emailed to any pager or email enabled communications device or can send SMS messages to any mobile device, PDA, Blackberry or cell phone which can receive emails.
Reminder Manager provides reminders from any email, calendar, tasks or contacts folder in any open mail store. This allows you to set reminders on items in any folder in your mailbox or PST file, secondary PST files or public folders. Reminders can be emailed to any pager or email enabled communications device.
With the Reminders Assistant plugin for Outlook you can set reminders for each and every Outlook folder including embedded ones.Reminders Assistant is an add-in for Microsoft Outlook. The Reminders Assistant component is one of the 18 addins included into the program MAPILab Toolbox.
For additional reminder tools, including utilities to send reminders by email, see Calendar Reminder Tools