I'd suggest replacing the power/volume flex cable. They are very delicate and sometimes it's difficult to tell if they are broken just by looking.
The only other thing I'd try is to adjust the two screws holding the power/lock button down. Sometimes if they are too tight it will restrict the use of the button. I will say though that this remedy is more successful for the volume up/down buttons than the lock button.
If you chose to replace it yourself, I'd suggest buying a couple new cables just in case. They can rip easily while they are being installed if you're not careful. Also, about 1 out of 10 will be dead in my experience so it's always nice to have another one just in case.
To fix it yourself, find a video on YouTube so you can see how it twists. The twists on installing that thing are a trip.