If the door is not closed properly, frost will form in the refrigerator. Close the door with a piece of paper between the door and the refrigerator, to determine whether the door closes properly. Pull at the piece of paper. If you feel resistance, the refrigerator door closes properly. If you feel no resistance, the door does not close properly.
Perform this test regularly on all four sides of the refrigerator door. If you find that the door does not close properly, check whether the door locking mechanism keeps the door properly shut.