Had similar issue with the lock button in my wife's windows phone. could only half pressed. That was because of some dirt got into the gap. tapped the phone on my palm 5 or 6 times and it started working fine, so that the dirt blocking the button got away. The same issue has come quite a few times and the tapping the phone on the palm worked all the times. May be you can try that. This happens only when me or my wife put the phone in our jeans.