Updates to a Booking/Event will automatically update a Document, unless it is locked however there are some common reasons why it has not done so.
If a Document is open on another tab, you would need to refresh the page, or re-open the Document for the change to take effect.
When a Line Item Document element doesn't update it is often due to the revenue category selected on the Menu Item Document - or the Menu Item added on the Booking does not have a Revenue Category matching that on a Document.
If a merge field is not updating, it may be due to an incorrect or old Merge field being used.