Most likely, you're dealing with some sort of software issue.

Experiencing issues with the volume buttons on your Android phone? If it’s software related, it may be tied to a bug with one of your phone's accessibility features. Google posted on its Android ...

Google is developing a new horizontal layout for Android’s volume slider that will likely appear when the device is in landscape mode. This new horizontal slider will be positioned at the top-center ...

We have received reports of volume keys not working as expected on Android devices that have Select to Speak, an ...