My iPod nano 7th gen with built in nike wont detect the sensor. It worked previously with no issues. Please help!

Nano is the latest 7th gen model. Have used Nike fitness a few times and without fault. When the female voice asks me to walk about to activate my sensor, it will not detect a sensor. This was not an issue when I previously used it. How does the sensor work? Where does it detect from? Can someone give me advice please?

Thanks