Some 3D models won't render

I have been creating 3D textures for a resource pack I was making and I was trialing a 3D model. I saved one version and it worked. I did more work to the model and made it larger and then added it as a different texture to see the difference between the two. but this 3d model didn't work. It just showed the purple and black square, missing texture. I don't understand at all why this happened!

In minecraft screenshot shows the previous model that works and the unrendered texture. The other image is what the unrendered texture should look like.