The likely cause of this is the Group Grades and Comments checkbox is Unmarked and/or the Display Grades In Grid is marked. When the Display Grades In Grid checkbox is marked, then only one Grading Period can be added to the Assessment Report Card. This is also true if the checkbox for Group Grades and Comments is deselected, only one Grading Period can be added to the Assessment Report Card.

To correct this, navigate to:
 
  1. onRecord > Performance > Grading > Report Card Builder
  2. Click Edit into the Report Card
  3. Unmark Display Grades In Grid checkbox if this is marked OR mark the Group Grades and Comments checkbox if this is unmarked
  4. Click Save or Save & Exit