Attendance is tracked automatically based on who actually joins the session — but that automatic tracking can miss a student in one common situation: if they joined through a public or shareable link rather than logging into their own dashboard and clicking into the session from there. That kind of join isn't picked up automatically.

If that's what happened, use the Mark Present checkbox on the attendance panel to correct it manually (see How attendance is tracked). If attendance looks wrong for a different reason, flag it to your admin.