Please see the article below about a big change that Apple made starting in iOS 8.3 when it comes to access Apps directories for apps that were downloaded from the App Store. If you need to copy a file to a folder of an app besides the Documents folder of an app that has iTunes File Sharing enabled, you would need to Jailbreak (hack iOS ) in order to do it. iExplorer will still let you transfer those files to your computer, but you just can’t transfer them to a device with the exception to the situation mentioned above.
App folders in iOS 8.3 are empty. Why is that?