Viewer circles shrinking when looking down in VR viewer

Hello, I am creating a 360 video app in unity for cardboard.

I have the video playing on a sphere but when you look down the circle displaying the footage shrinks, so you only have a small field of vision in each eye.

I have tried downloading the Google Cardboard SDK for unity to see if the prefabs gvrmain camera resolves this problem, however, this camera does not appear in the prefabs.

I am very stuck. I would greatly appreciate any advice you could offer me.