Ads don't run if game started with no internet access (and then turn internet on)

Hi

I'm having a small issue with Unity Ads,

I'm using Unity 5.2 and the integrated service,

If I start the game with no internet access, and then turn on data (or wifi), i can't get the Ads to run, when the Ad button is pushed.
Is there something really simple I’m missing?

I tried adding `Advertisement.Initialize ("mygameID", true);` to my button code, but it didn’t work.

Any help would be appreciated

Thanks