Gradebook

I hope I can explain the problem clearly.  In a course of mine students are divided into three groups.  they have three assignments cd 01, cd 02, and cd 03 under the catagory Chinese Dictation.  When I tried to view how each group is doing under the detail catagory view by selecting a group name(eg. Miss Tung).
After I selected a group name, instead of what I expected, the screen refresh and jumped up one level to "All grades by catagory" with the correct group of student displayed.

My question is am I able to view by group in the detail catagory view(first screen capture)?  If not, what is the use of the combo box?  If yes, what have I done wrong?