I have Nexus 5x using Android N.
I had noticed that I whenever I switched to Google app home screen or the phone's home screen,
then in the search box, the phrase "Say "Ok Google"" would only appear momentarily and then would switch to "Search". This caused the voice search never to start whenever I said "Ok Google".
I found out that the issue was caused by Keepass2Android Autofill service:
by disabling it in the phone's Settings|Accessibility
then the search box would always show "Say "OK Google"" again.
I had noticed that I whenever I switched to Google app home screen or the phone's home screen,
then in the search box, the phrase "Say "Ok Google"" would only appear momentarily and then would switch to "Search". This caused the voice search never to start whenever I said "Ok Google".
I found out that the issue was caused by Keepass2Android Autofill service:
by disabling it in the phone's Settings|Accessibility
then the search box would always show "Say "OK Google"" again.