Have you enabled depth testing, and have a depth buffer?

glEnable(GL_DEPTH_TEST);

GLint depth_bits;
glGetIntegerv(GL_DEPTH_BITS, &depth_bits);
depth_bits should be >0.

If your code is derived from Apple's sample EAGLView, search for "USE_DEPTH_BUFFER" and set it to 1 to create a depth buffer.

Did you request a depth buffer and enable depth testing? Did you turn on back-face culling?

Thanks for your help.

I managed to fix my problem, forgot to enable depth testing; however, it seems as if the back face of the triangle is missing. You can see this in the screenshots I posted, I thought they were caused by the same issue, but I guess not. It's as if it isn't being drawn.

EDIT: Stupid me, I fixed my problem. Was specifying the vertex count as 9, not 12.