Unable to make apps use mobile/cellular data

Settings > Mobile Data > App > turn the option on/off

I used to be able to toggle between on/off but now i don't seem to be able to.

Take Snapchat as an example (but this applies to all) - i have it set to not use mobile data. If i set it to use mobile data & then exit then when i open the app it wont use mobile data still. Go back to settings & it's as though i haven't changed it.

Same for when i want to change from using mobile data to not.

iPhone 6S on iOS 9.0.1 not jailbroken.