How to Restore Lost Placeholders in PowerPoint 2013
Call up the Master with the missing placeholder. In the Slide Master tab on the Ribbon, click the Master Layout button (in the Master Layout group).
This step calls up the Master Layout dialog box.
The Master Layout dialog box is one of the strangest dialog boxes you encounter. If you summon it for a Master that still has all its placeholders, all the check boxes on the Master Layout dialog box are grayed out.
So, all you can do is look at the controls, grunt, scratch your head, and then click OK to dismiss the seemingly useless dialog box. However, if you have deleted one or more placeholders, the check boxes for the missing placeholders are available.