The way the Assignment Calendar works is it will show you just the assignments that are assigned to ALL the checked names. So if you have checked all of your student's names, the only thing displayed on the calendar will be assignments that each of your students have in common.
There's just not enough room on the calendar to display all of your students' individual assignments at the same time, although some parents get around this by including themselves in all assignments. Then uncheck all but the parent's name to see everything. This would be workable if you don't have too many assignments and just want an overview of everything all at once.
In order to see just one student's assignments, make sure that only his or her name is checked. If you want to assign a lesson plan to multiple students at once, make sure that all of their names are checked before assigning.