"Ok google" doesn't work Device: LG G3 F400k
Rom: CM12 Nightly (12.1-20151004-NIGHTLY-F400)
Google App version: 5.3.23.19.arm
Google services version: 8.1.15 (2250156-230)
Google Play store version: 5.9.12

Everything seems up to date. But I don't have the options to use Ok Google on other screens/apps. And the hotword won't even work in the app itself.
Screenshot:

Click to enlarge
I had the same issue with my device as well.

Device: SM-G360T
Android version: 5.1.1
Google App version: 5.13.31.21.arm
Google services version: 9.0.83 (234-121911109)
Google Play store version: 6.5.08

My solution was to download the Google Now Launcher (version 1.3.large). It replaces your current launcher, but I thought it was a good exchange. Warning! Replacing your current launcher will change your current home screen applications layout, but you can always uninstall Google Now Launcher from the Applications Manager to return to your old launcher.