Beginning with iOS 8.3, Apple has blocked access to most of the Apps directory for apps downloaded from the App Store, and the same limited access is in their latest iOS 9 release as well.
In the meantime, as a workaround, to transfer app files from your iPhone or iPad to your computer, you could go to the iTunes Backups section instead since iTunes backs up both the Documents and Library subfolders of every app downloaded from the App Store. Then, just find the app in the list of folders (every app will be listed as their bundle ID in a reverse domain format (com.developer.AppName)), and you can simply drag/drop any files to your computer or right-click and choose Export to folder.
Only the developer of an iOS app can enable iTunes File Sharing, so we encourage all of you to contact the developer of any iOS apps that you want to be able to access and ask them to release an update to enable file sharing. It's extremely easy for them to do.
The only apps that aren't affected by this in iOS 8.3-8.4 and 9 are ones installed via an Enterprise app certificate, TestFlight, or via Xcode on a Mac. The files in those apps installed outside the App Store can still be fully accessed in the Apps section of iBrowse with iOS 8.3-8.4 and 9 devices.
There is now a Jailbreak for iOS 8.3 and 8.4, so if you Jailbreak and install Apple File Conduit "2" from Cydia, that will give you access to the Root folder of iOS which means full access to all app folders and files. You can find out more at the Jailbreak site below: