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.
More Information
For additional reminder tools, including utilities to send reminders by email, see Calendar Reminder Tools
your add ON does not work for the public folders!
What version of Outlook and which addin are you using?