A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
When you view the Slide Master, you should see a pushpin icon to the left of the slide master. This indicates that the master will be preserved when you save the file. If that icon is not visible, please right-click on the slide master and choose Preserve Master.
Rarely, I see presentation where even preserved masters disappear. You should be able to prevent this by adding at least one slide from each slide master to the presentation. PowerPoint will not delete masters that are actively being used by slides.