Outlook 2007, dont’always see tasks & flagged e-mails

Some days I will get notified of scheduled meetings and flagged e-mails and other days no reminders appear. When I don’t receive any reminders, there is an error message where the tasks and flagged e-mails would reside: “The operation failed. An object could not be found.” When I click on the tasks icon (under the calendar on the left), I get “folder could not be found”