I no longer want two master pages
Microsoft Publisher will now use the right background for all the pages of your publication.
An object on the master page appears on only some of the pages
The object on the master page might be hidden by something on the foreground. If it's hidden by something you want to be able to see through, make that foreground object transparent.
The change I made on the master page appears on only some of my pages
Your publication probably has facing pages, as a book does. This means it has one master page for the left pages and one for the right pages. You need to make the change to both master pages.