If the volume up/down buttons are working normally, you probably have a defective cable or a "micro-rip" in the cable. Best way you can tell is by using a multimeter to test the power button.
In my experience, poor quality power-mute-volume flex cables are a dime a dozen. I often have to try 4 or 5 before getting one that has all the functions. So much so that I now plug and test them before installing them.