Tap on the Options. Check the checkbox for "Background Clear". It the icon for Background Clear show a padlock, that means you haven't made the In-App Purchase. Please tap on the padlock to purchase through the App Store. For previously In-App Purchase customer, please restore the purchase for free.
Click on the Home button to exit out of the app.
Tap on Settings app
Choose General -> Background App Refresh
Make sure it is turned on on the top
Scroll down to Clearboard section. Make sure it is turned on
Note: Make sure that you don't remove Clearboard from the multitasking list (when you double click the home button, Clearboard should be in the list.)