Periods and deadlines come from the obligation's settings, so start there. Open the entity and the obligation.
What to check
Start and end date: periods are only created while the obligation is active.
Filing periodicity: it sets how often a return is filed and when each one is due, for example monthly, with the return due 20 days after each period.
The period itself: if only one period has an unexpected deadline, open that period and check its filing due date. Select the edit icon. If Override is switched on, the due date was changed for this period only.