The badge requires each individual instance quest to be cleared while in Tomb of the Beguiler. Its possible to miss the quest completion trigger if you were dead/not in the dungeon when that particular instance quest was cleared. (Example, killing Serket Guardian clears one of the instance quests then gives you another instance quest). Should probably be changed to just require completion of either the last instance quest or the quest outside asking you to defeat Ngozi the Beguiler to prevent this.