Frank C. answered 03/06/21
Pro Photographer - Teaches Camera Basics & City Street Photography
Chloe,
Did you make sure that you selected the oversized thumbnail when inserting the footer in your Slide Master? This is the thumbnail that sits at the very top and is the Slide Master creator.
PowerPoint defaults to the Title Slide Layout thumbnail when you first go into the Slide Master View, not the Slide Master itself - you have to go to the very top of the thumbnail section and choose that oversized thumbnail, which is the Slide Master creator itself.
(Sort of like choosing a parking space; putting your car into reverse; then begin working on the details.)
Frank C.