I absolutely believe you if you say it doesn't work for you most of the time. But I have zero reason to believe that would be true for others. If someone else says it works for them almost all the time, would you call them a lier or on the other side of the coin think that means Hey Cortana works for everyone? You are one data point.
after using it on a cheap 17" laptop I realized how important is the actual hardware for it to work, we normally take it for granted on our phones since they are.. after all... "phones" they should have best in class audio, but that doesn't apply to other devices, im not saying the mighty 1520 has bad mics, but maybe this one isn't that good... you should check that, record a message and then compare it to another phone, preferably a similar lumia too
after trying it on my main computer(a desktop one) "hey cortana" works even when im in the bathroom, with 2 fans on in the room(don't ask) and music on
Really cause mine has worked 98% of the time and I LOVE it
I think the answer is here
https://www.qualcomm.com/news/onq/2...akes-mobile-world-snapdragon-voice-activation
Hey Cortana uses a HW feature in the Qualcomm platform. This voice activation is likely to be much more basic than the then off line voice analysis. Your voice is after the "Hey Cortana" detection is sent to a server (if you enabled it) which can much more sophisticated voice analysis. I can imagine if your accent is a bit off or you have slurred speech, the HW detection can fail.