When your iPad is playing a loud sound from an alert or from a certain application you might want to use the side volumn button to lower it down. However you when press on the side button nothing has happen. To fix the issue you will need to enable the "Change With Button" from the setting screen. Here's how:
1. Go to the Home screen.
2. Then tap on Settings.
3. Select General.
4. Now select Sounds.
5. Then slide the Change with Buttons to the On position.
Note that when playing video or mp3 music file the button will not work.