Whenever you make a change to a field on an item, the link between that item and its
Standard Value is effectively broken. Since the
Final Renderings field which are used for Shared/Final Layout are just regular Sitecore fields, the same applies to the
Presentation Details which you set on the Standard Values, i.e. if you have made a change to the item by adding/removing renderings (for example) then you have broken the link.
In order to restore the link back to the Standard Values you can
Reset the layout fields. Be sure to select the Renderings and/or Final Renderings fields as needed.
Word of warning, it will reset the layout and remove any additional Renderings that may have been added by you users. If you need them on the page, you will need to add them back in.
From the Experience Editor:
From the Content Editor:
You could also write a PowerShell script to Reset-Layout of items if you have a lot of items.