If your are using a Cool Promo White Label installed in your own app, you need to set it up so it meets the new width of Facebook tabs, which is 810 px.
If the app is new, you won't need to do anything, as it's set up by default. If you are reusing the app, follow these steps.
2. Click on "Settings" and go to Page Tab.
3. You must check that Yes is selected in the Wide Page Tab? section. If not, just modify it.
4. Save your new application.