Over the past month or so, some Android users have been complaining that the "OK Google" activation has not been working as triggered.
It turns out the reason it is not working for some is because of the "Trusted Voice" security setting.
After tons and tons of posts in the Google Web Search Help forum, AJ from Google thinks he got to the bottom of the issue. He said:
The reason we have limited functionality from a locked screen (without Trusted Voice enabled) is to protect users’ security on their devices. For example, if anyone was able to send a text or make a call from the lockscreen without unlocking, it would expose the user's’ personal information insecurely.However, if you’d like to be able to make calls or send texts from a locked screen, Trusted Voice must be enabled. The way to enable it is to go to Settings > Voice > "OK Google" detection > toggle 'Trusted Voice' on.
Please note that there are several devices that are not compatible with ‘Trusted Voice’: Nexus 6, Moto X, and Galaxy Note 4 and Note Edge.
You can learn more about how to configure this setting over here.
It doesn't seem everyone is happy about this.
Forum discussion at Google Web Search Help.