Why do the treads on this tank disappear when switched to full 3D view?

Why do the right-side tracks on this tank disappear when I switch from default view to full 3D view.

This happens in both blender render and cycles render.

Also, when I try to render a turntable animation, the tracks are missing.

check the outline panel… the object eye might be closed !!!

You might also need to check that if the tank track uses a mirror modifier that you have not disabled rendering for it: