Outline not showing + and -

 When I use the Outline function in the data tab, I select a couple of columns. But the + and - sign that should come up above the column does not. I only get a small mark at one of the columns and I have circled it in the image I have attached. How do I get the default view back? I have already checked File -> Options -> Advanced -> Show options for this workbook / worksheet: Show outline symbols if an outline has been applied -> checkmark!