To ensure that your device itself is ready to receive push notifications you must restart the Mail.app. To restart the Mail.app, perform the following steps:
- Open the Mail.app on your iOS device.
- Double-click the Home button.
- Swipe up to close the Mail.app.
- In the Mail.app, swipe down (to perform a manual fetch).
Note: If the above steps do not successfully initiate push notifications, you may need to restart your device.
For more information about how to close an app, read Apple's Force an app to close