Grade sheet not allowing a single student's grade to be marked COMPLETE

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In my course, I have students submit a "contract" as an assignment which is not assigned a point value, so it is to used to calculate the final grade. However, I do mark the assignment as COMPLETE once it is received. Once the assignment is marked COMPLETE, a green check mark appears for the assignment for all but ONE student. Although the student's contract has been received and marked complete, a "file" appears in the grade sheet, and the entire cell is green, but will not display the green checkmark. Any suggestions?