I had the same problem with one of my buttons. All I did was pushed it repeatedly, and it started working right again lol.May not be the smartest thing to do. But it worked for me..But then again, may just be internal probs
I agree it's probably dirty. Opening and cleaning it is an option but they can be tricky to put back together. An easier thing to do is put just a little hand sanitizer on the button and press it repeatedly until it works better. This wouldn't be my first choice but if you're afraid to take it apart it'll work.