Problem Pairing Edge 500 and garmin cadence sensor

So as the title says I'm having a problem with my Garmin Edge being unable to detect the cadence sensor. The cadence sensor is set up correctly as it flashes at the right time,e.g when the crank arm passes the sensor and when the spoke magnet passes so I know its not the setup. Does anyone know what I have done wrong or have I just missed a step to pair the two together. Any help is much appreciated. Thanks
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Then start moving the cranks/wheel, and the unit should say 'speed/cadence unit detected press start to begin training' or something like that.