Every once in a while I have the same issue, what is interesting is that the button has two "states" - touch and depressed. You can see this in the Service menu, when you touch the button but don't depress - it has an indicator that it's being touched.
What caught me as interesting is that if the button doesn't have this "touched" indicator, it physically cannot be depressed. Say for example you use a pen that won't "conduct" and trigger the "touch" state - the button seems to be physically blocked from being depressed. First off, this is really interesting to me as there seems to be a physical limiter in the switch that prevents the button from moving unless it senses touch. I assume though that this is where I'm having trouble - that every once in a while my finger isn't landing in the correct position to trigger the "touch" state or that something is preventing that touch from being registered.
I don't think this is good or excusable but if you're consistently having issues, I'd go into the Service menu and play around with just touching the buttons but not depressing and seeing if your touch state is being captured when and how you would expect.
