Verify that the reminders you are looking for are present in the default calendar, because reminders present in a non-default .pst file will not pop up.

If the reminders are present in default .pst file and still reminders do not pop up, try the following:

Exit Outlook, click Start; and type outlook /cleanreminders in the Search Programs and Files box and press the enter key and check if it helps.

If Outlook /cleanreminders did not work, try Outlook /resetfolders.