When you edit the Close Booking date and/or time of an event, after clicking Update, a pop up box will appear.
You have 2 options:
Apply to Event ONLY
Or
Apply to Event and all Items (CLASSES)
If you want every class to have the new closing date then select the All Items (CLASSES) option.
To check that the close booking date is now correctly applied you can preview the event.
To do this, click on the Preview Event icon, this takes you to the public view of the event.