Shapes that are invisible on the screen appear on the printout or vice versa, shapes with fill color may be invisible on the printout. Shapes show an outline only on the printout.
Different settings in View → Color/Grayscale and Print dialog → Color/grayscale may cause a discrepancy between the view on the screen and the printout. For example, shapes that are invisible on the screen in Color mode (e.g., because their fill and outline match the background) may be visible on the printout if Grayscale or Pure Black and White printing is selected. On the other hand, a shape with a fill color that is different from background may be invisible on a grayscale printout because its grayscale value and that of its background coincide.
Make the settings View → Color/Grayscale and Print dialog → Color/grayscale match.
For details please see Microsoft's PowerPoint documentation.