I've seen this on several iPhone 5's so far. Usually the button still "clicks" fine, but just doesnt function correctly. If its still under warranty, you can take it to an Apple store and they will replace. Its a part issue. If its not under warranty for some reason (they come with 1 year)....Then you need to replace the power/vol/mute flex cable. This is about a $8 part online just about anywhere. Its rather difficult to replace.... it takes time and patience to install correctly. But it will resolve the issue as well.
Hope this helps!