In some instances, you can blast the button edges with canned air, which may clean them enough to remove some of the carbon deposits.
Other than that, replacement is quite expensive and may not be worth the trouble. I have the same problem with my main power button, and it must be replaced. But, this will cost me about $150 just for the parts and labor to do the job. In order to get to it, you must remove every circuit board from the keyboard. Really poor design.
Good luck,
Gary