In the default render script, z must be between -1 and 1. Values outside of that will not be rendered. You can modify the render script if you wish to change the range.
Z values are cumulative. If you have a game object with z 0.5, and inside that game object a sprite with z of 0.75, then the sprite will not be rendered (0.5+0.75=1.25 > 1).