Some exams ("HEM","G") below are not added to the exam line, I don't understand why they don't be printed

I don't understand why my program is skipping the two exams mentioned above.I really tried to think of solutions to my problem but unfortunately I couldn't

    public class Main {
        static String exams = "";
    
        public static void main(String args[]) {
            exams = addExam("TSH");
            exams = addExam("T4L");
            exams = addExam("HEMOG");
            exams = addExam("COL");
            exams = addExam("HEM");
            exams = addExam("TRIG");
            exams = addExam("G");
            exams = addExam("GGT");
            System.out.println(exams);
        }
    
        private static String addExam(String exm) {
            if (exams.indexOf(exm) < 0) {
                exams += fillVoido(exm);
            }
            return exams;
        }
    
        private static String fillVoid(String s) {
            String back = s;
            if (null != s && s.length() < 6) {
                for (int i = s.length(); i < 6; i++) {
                    back += " ";
                }
            }
            return back;
        }
    }