Applying Feedback Master does not override the font size on actual slide

Without seeing the file, it's difficult to know what the person did. If the user added content to the slide using the layouts within the slide master, then those objects should change if the layout is reapplied. 

If it's not changing, then odds are some of the content isn't associated to the slide master layout. 

If you want to share what you're working with, then it may be easier to offer some tips to help speed up your work.