This usually indicates a permissions issue with the Administrators Only security category.  If there is an override for Site Visitors, ensure that they are not set to View Content in the Admin category.  To check this, follow the steps below:
  1. Setup>Security Categories
  2. Click Administrators only
  3. Click Category Permissions
  4. Select Content Viewing from the drop down and click Choose
  5. Under Groups being overridden, click "Show all groups"
  6. Look for Site Visitors and make sure that the option for "Cannot View Content in the category Administrators only" is marked
  7. Click Save