Texture on plane not displaying properly

I have been working on a 3D interface for my game and cannot figure out why some of my planes wont display over one another.

Plane 1 (the big circle) should be underneath Plane 2 (the green arrow) but it doesn't display right when the camera is not directly over it.

As you can see when my camera is directly over the green arrow it shows above the circle but otherwise it doesn't. Can somebody explain why this happens (if it is a bug) or how to fix it.