I should also mention that unlike the original Cardboard viewer, this thing has a strong tendency to slip out of correct alignment if you don't hold it carefully, since it's just clipped onto the middle of your phone screen. This can happen before you really realize it, so that the symptom is that you can't fuse the images (something that happens to me a lot in real life because of a physical defect in my eyes, so my first assumption isn't that the viewer slipped).