"Ok google" used to work when phone was off

[Android Central Question]

On my old nexus 5x the phone used to wake up when I said "ok google" even when the screen was black. I've since switched service providers and got a new Asus phone. This phone does not respond once it is asleep. "ok google" does work after pressing the home button to wake it up. I need this feature because I have a disability and it is an extra security for me. If I fall I need to be able to tell the phone to call without pressing any buttons. I've gone through the settings but have not been able to figure this out. Any ideas? Thanks!