The double tap to wake works after the gyroscope has been initiated and recognises that the phone has moved. So if it's just sitting flat on your desk and you double tap, it won't wake up the screen. You have lift the phone up (or move it a bit), the gyroscope recognises that the phone has moved, then it activates the touch capacitor, then double tap to wake will work.
May seem clumsy in implementation, however not having the capacitor on constantly saves battery life.
Also the M8 has among the best touch sensitivity of any phone on the market:
http://blog.gsmarena.com/the-htc-one-m8-is-king-of-touch-screen-response-time/
Sent from my HTC One M8 using Tapatalk