The system looks at underlying attendance entered for the duration of the class to determine which icon to display on the class in the SIP.
Why is the attendance icon not appearing on the class?
If there is no icon appearing on a class this typically indicates there is no attendance entered for the class for it's duration. To check if this is the case, open the Attendance pane in the SIP and click the 'Time-based' button. This will display the underlying attendance - if required click on the dropdown and select 'This year' till to display the date for the class in question. If you can see gaps then attendance hasn't been recorded for that period of time.
Managing attendance with roll classes verses subject classes
If students have been enrolled in two sets of classes at the same time, for example Roll classes (Day roll, or AM/PM roll) and Subject classes (Period-based) it's important to either mark attendance using the ROLL classes, or mark attendance for every single period based class (assuming the full day is timetabled). This ensures full days attendance is captured.
Both classes can be used to mark attendance providing the roll class is marked first, then any adjustments made using subject classes (for example if a student doesn't attend on of the period-based classes they can be marked absent using this class). However, the suitability of the option depends on school policy.