Share your experience!
Which Xperia do you have?
Open the app drawer > look for Google > Swipe left to right > Settings > Voice > OK Google detection
you sould see
From the Google app
From any screen
Personal results
OK Google Voice Model
Retrain voice model
Delete voice model
Did you update the phone to lollipop now, 5.1.1? If yes, this is your answer:
https://productforums.google.com/forum/#!topic/websearch/ROuqhFr2yvk
M2 is seen as a low RAM device, that is why some features are missing.
Keep us updated.