[Q] "ok google" always on fails after first run topic

I have some problems keeping "ok google" always-on function working in 910c Note 4. After enabling it for working in lock screen it wakes the phone from lock screen or with the screen locked and off but after that first time it stops working. It continues working but only from home screen and the option to enable it again turns grayed. I have to reboot to have the chance for enabling it again but only for one time. I have tried turning off s-voice but same behaviour.

Any ideas?

It is Spanish as the default language that is supported in the latest version.