Cellular data keep turning off

Im using iphone 6. When my iPhone still using ios 8, I turn off cellular data for some applications to prevent them consuming the cellular data. Now after I upgrade to IOS 9, I try to turn on the cellular data for the application that been turned off (facebook, youtube,etc). But everytime I turn it on and go back to the settings, the cellular data keeps turning off. Then I try to turn off cellular data for application that been turned on all the time (Mail,etc) and turn it back on and it works. So the problem is, now I can't turn on the cellular data for some applications that been turned off since I use IOS 8. Anyone has the solution? Thankyou