Cardboard view vr on Iphone

I want to use Cardboard to view in VR mode on IOS. But when you click VR icon, sometimes such a message appears, what is happen, Could you help me to fix this problem?
Capture

Do I understand correctly that you can successfully enter the VR mode, but not at each attempt? How many triangles does your scene have (you can check it at the bottom of the “Objects” tab in the editor)?