I'm also having the same problem with my S4, however after reading the replies, I believe mine may be a bit different. I have been using the "OK Google" from any screen for months now. I used it so much I installed Google chrome on my PC and tablet and enabled the command on all three.In fact, it's become quite funny, when I have all 3 devices on, I'll say OK Google and they all wake up. Recently though it doesn't want to work every time, like it was. I'm hoping someone has the answer, or I'll keep stumbling around without, the assistance of my phone and the web, to get me the answers to the, oh so many questions I have just going through the day