When you edit a particular Ad Creative, the system automatically assumes that you want it to be separate from the default design for the entire catalog. Any changes you make to the Master Template will not reflect on any Ad Creative that you have edited.
If you want a customized Ad Creative to take on the design of the Master Template, click on the Reset link under the Ad Creative.
Any edits you've made to that particular creative will be removed and it will revert to the design of the Master Template.