Volume up button does not work as camera shutter

I have an iPhone 5 with iOS 9.2.1 (13D15). The volume up button does not work on it as a camera shutter. I think it used to, and my wife's iPhone 6 with same iOS version works fine. This is a problem, because I am trying to use this phone with a selfie stick, which triggers photos by simulating the volume up button press, so it does not work for me at all. The physical button works - I can control volume settings with it. And when I try to use the selfie stick shutter, I get the volume up display on the phone, so I can see that the signal from the selfie stick is received properly by the phone. And the selfie stick works with my other phone that does not have a problem with the volume up button.