I have tried soft reset, but by holding down the power button for 10 secs. Are there several versions of soft reset? I'll try the vol down version.
I have tried a full factory reset as stated. Did that after installing Denim because I had the slow rotation bug after camera use that many get with Denim (the hard reset solved that). I have just got my phone and so that's effectively 2 hard resets. I can't keep resetting, I just don't have enough life to spend it fannying with my phone all the time.
Syncing is fine, and I certainly don't have that awful Win10 installed on anything (and it never will be if they don't keep the legacy interface out of my immersive modern interface). So yes, both Cortana and Hey Cortana are effectively 'disabled' on my tablet and PC as it is not available on Win8.1. Could it be that Hey Cortana only works if Win10 is installed on my other devices? But surely there'd be loads of posts about this?
Battery saver is set to come on at 20% and rarely does before I top up. Cortana is set to stay on even in battery saver mode though. Is there a separate setting for keeping the Hey Cortana feature on during battery save? I can't find it. Still, I can confirm it will switch off even if battery saver has never engaged between checks.
What concerns me is that, after a hard reset, this keeps happening and seemingly only to me. So either my phone is broken or I'm doing something wrong here.