Unity app built with google cardboard requires to go out of app then back in for cardboard tracking to work.

I recently built an android app for use with google cardboard, using the cardboard unity SDK. When the cardboard scene is first loaded, the viewport is frozen (in stereoscopic), on the first frame of the level. Waiting and moving phone does nothing. Hit the back button, or recent apps buton on the phone, then go back in to the app, and tracking starts, and the screen unfreezes.

Has anyone else had this problem? If not, can anyone atleast hazard a guess as to what momentarily leaving the app does to set head tracking working?

Thanks