Seeing Second level Sub-tasks

The system allows creating a list of sub-tasks within (under) a specific sub-task. When I see my day plan, I see the 1st level sub-task as a checkbox. I can check (to mark completion) or click again to uncheck. That's it. I can never see what's under that.

Is what I am doing not supposed to be done that way? Or is it a bug?

Please reclassify Post In as appropriate.