When I moved to a new laptop with Office 2016, I no longer saw an option "Personal" when creating a new presentation. I checked Documents > Custom Office Templates, and they were all there. Why did Powerpoint not see them?
It turns out that by default, Powerpoint 2016 does not look for them there. It looks under its program folder, of all places, which is a horrible place to keep any files that you wish to migrate from computer to computer when you upgrade. Fortunately, there is an easy fix.
On the menu bar, click File > Options, and then choose "Save" from the options in the left column of the resulting window.
Next, in the box "Default personal templates location", enter the location you where you want to store your personal templates. For me, that is "C:\Users\matthews\Documents\Custom Office Templates\". You can search your drive to find the proper location and its full path name.
Choose "OK", and you are done.
The next time you create a new Powerpoint, you will once again have a "Personal" option for templates, and your templates will be visible when you select that option.