There are dozens and dozens of complaints from searchers in the Google Web Search Forums that the OKAY Google detection feature has stopped working.
With Google mobile devices or any device that supports "Ok Google" commands, when you say "okay Google," Google will start listening to your voice and turn that into a search query. It would simply trigger voice search in Google.
But many are saying that all of a sudden, the "okay google" command has stopped working. They can say "okay Google" a hundred different ways but it simply does not bring up the voice search prompt. Yes, they have the settings on, but still, it does not work.
AJ from Google is working through debugging the issue so that he can either help the searchers fix their devices or report back to the engineers with a bug in the software, so they can fix it going forward.
So far, no resolution has been made and more and more searchers are noticing the issue.
Forum discussion at Google Web Search Forums.