Yes! My Octave buttons (both up and down) have similar hit and miss behavior too.
It seems they are much more responsive and reliable when pressing the "inside" edge of the buttons. The further out on the buttons I push, the less responsive they become. (Even the very center of the button does not work sometimes)
I've just learned over time to always press the inside edge of the octave buttons and give it a decent 'press' rather than a 'tap'
I don't think they are faulty, I think the buttons are just too long with too much play. Once I learned the behavior of them, they've never failed and always work when I press them as described above. it was a bit annoying for sure though when I first got the instrument and was always pressing the 'middle' of the button and it not working.
design flaw!