Reboot. For some reason, after the phone is connected to bluetooth device, this happens. Maybe turning off bluetooth and back on will also resolve it, I haven't tried that yet.
I've noticed the same thing. Used to work on the lock screen as well (when plugged in), and it no longer does this. It activates when you say OK Google, but it won't give you the answer (or do whatever you asked it to do) until you unlock the device.