I'm not sure what's going on. Just to be sure, go to the Play Store and search for the Google app, which is the square blue icon with the Google G on it. Make sure it says installed. If it says update then go ahead and do so. If you do have to update, try another Okay Google voice search. If that doesn't work, try going into the Google app itself then either slide the menu out from left to right or press the menu button in the Google search box. Go to Settings > Voice > "OK Google" Detection then delete the voice model. Then just go back in and select train voice model so Google can recognize your voice. If after all this and you still have issues I'd suggest getting in contact with Google.
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