In OpenGL, I display a simple model. When I enable the depth buffer,
glEnable(GL_CULL_FACE);
glEnable(GL_DEPTH_TEST);
glDepthMask(GL_TRUE);
If the following answers above do not work try the following:
GL_CULL_FACE enabled check the winding order of your model's vertices. (LH) vs (RH). GL_DEPTH_TEST is enabled; then check the planes to your frustum against your model view projection matrix along with the camera's position & view direction.GL_BLEND is enabled; check the rendering order of your objects in comparison to the z-buffer.