Why wont Nike + iPod sensor activate?

I thought i'd configure this to see exactly how much exercise i i am during during my walks at work however i have had no luck in setting up the sensor.

When i went to go on one of my walks i went to configure it, all it showed was the message "walk around to activate your sensor" and i notice my bluetooth turns on and off all the time. my walks are 30 minutes and half way through it was still doing the same thing.

my iPhone is version 8.1.2 and it's an iPhone 5/5S. i am unsure which however i got it from my mother last year and i got This cover which fits perfectly however i can rule out it interfering as i pre-ordered the cover months in advance after getting my phone and i was still unable to activate the sensor (i retained my old iphone to be a music player but it's cover did not fit my new phone).