This may be due to a permissions issue. However, if it is not, these steps should help:
  1. On the tabs across the top, click on the last tab to the right that has a plus (+) symbol.
  2. Cick on the "Customize My Tabs" at the top right.
  3. Select the missing tab from the left and click on the add arrow pointing right.
  4. Click the "Save" button.