Use this repair guide to clean sticky buttons.
Use a slightly damp toothbrush to scrape in between the desired sticky buttons. You can also use a bit of isopropyl alcohol.
I just used this method to clean sticky buttons on a wireless Xbox 360 controller. The Back, Xbox Guide, Start, X, Y, B, and A buttons were all sticky and unresponsive.
I removed the battery pack and followed the steps, and it worked like a charm. All of the buttons work just fine now except for the Xbox Guide button, which isn't really a big deal anyway.
This method works great. Thanks for making this guide, Adam.
Wow. I thought I was going to have to buy a new Wii U Pro Controller, but this fixed my sticky B button. So simple. Thanks for saving me $50, friend.