Well if the problem occurs after a factory reset, you may have an issue requiring warranty service.
Was your device purchased brand new direct from sprint or did you purchase second hand?
Once you reboot the device, the volume button works yes? How much time passes before it's unresponsive?
Can you still use the on screen volume adjustments during the time the button is not responsive?
Does the down button work when the up button is not? :-)
Has this issue been a problem since day one of you using the device?