Are you "save as"ing it to the same name? You can’t do that, especially if it is a “default” style installed with the program. Those live in a protected folder with the program. The ones you save go to a User folder. If there are two the same name in each of the places, Endnote usually uses the default version.
Then you need to go into your document and find and select the edited version to assign on the endnote tab/ribbon.