Yesterday my wacom pen stylus just fell from my hand right onto the tip. I don't know how it slipped but the button jumped and I could see through the button holes the PCB. I unscrewed the pen tip and removed the rubber grip. I cleaned it on the inside from collected dust (just like you would get with the wheel mouse), and put everything back in.
To put everything back in I started with the rubber grip which slided over the plastic pen, then put the button back into the holes and pressed gently until I heard a click which meant the buttons became stable. At last I screwed the pen tip back and now my pen it's functioning normal. If there are people clumsy like me hope this post will help you solve your problem. I added a photo to visualize it better :) Good luck everyone!
For more info please check my other post where I fixed a different problem with the button and it's all explained with the aid of photos.