How Do I Turn On Automatic Downloads For My iPhone And iPad?
To turn on Automatic Downloads on your iPhone or iPad, launch the Settings app and select Store. Then select which type of purchases you would like to enable for automatic downloads (music, apps, books are the options).
You should also enable Automatic Downloads on your Mac. You can do that by launching iTunes on your Mac and going to iTunes > Preferences. Select the Store tab and then check the box for the types of downloads that you would like to enable.