I can't see completed tasks

When a task has been marked as "Completed" it won't show in any of the task views. This is so that you can focus on the tasks you still need to complete.
However, if you would like to see your completed tasks all you need to do is set your "Complete" filter to "Yes" as follows:
When you have chosen to show tasks with a "Completed" status they will show in whatever tasks view you are looking at.