Currently, you can't see Sub-Tasks split out at the "Board" view. They are rolled up into the parent task. The reasoning I was given is that subtasks don't live in the space or project they are created, but rather within the task itself and they are given a "ghost tag" to help you identify where the parent task is.
The work around provided to me by Wrike is to go in and tag the space/project and it will show up then. While this works, it isn't convenient and adds (what I believe to be) unnecessary steps. Based on my experience with other similar project management boards, tasks and sub tasks are visible on the board automatically and you can filter out tasks from sub tasks if you want to just see one or the other.
I see little to any need to use "ghost tags" to fulfill this need. We should be able to see all tasks and sub tasks on a board to better understand where ALL tasks are in the workflow. We shouldn't have to jump through hoops to make this happen.