iPod Nano Nike + issue: Nike + not recording distance while running

Can anyone advise me?

I have an iPod Nano 6th Gen and every time I go for a run, and turn on Nike +, it measures the time but doesn't record the distance.

I've already connected it to iTunes, updated the Nike + software and I've tried to calibrate it through the iPod's settings.

any advice in how to fix this would be fab!

thanks