My phone had been sitting on my desk, and I haven't touched it for at least two hours. I said 'OK Google' to it, and it beeped right away.
Btw, I don't understand people who are having problems with their gadget and posted by titling their thread as if it's a universal issue.
In this particular case it's not just the title. The question OP is asking is the generic "Is there any way to make "OK Google" work more consistently from any screen?". Well how do we answer that besides with 'nothing, because it works just fine'?