iTunes can be used to view more information regarding your Apple device. To do this plug your device into your computer and open iTunes. You should see the name of your iPhone, iPod or iPad appear at the top of iTunes below the search bar.
Click on your device and then scroll down to check that "Automatically Sync when this device is connected" is checked.
You also need to make sure that your device is running the most up to date version of iOS, and at least version 6.0 or higher by tapping on 'Check for Updates' on the same screen. Apple will prompt you and upgrade your software if necessary.
Another reason that the app may be unable to sync could be that your iPad, iPhone or iPod Touch does not have enough spare capacity. Try removing some Apps or Video that you no longer use to free up more space on your device. You can see how much free-space you have at the bottom of the same device summary tab.