Using PPT 2016 on a PC. Frustrating!!!

This has happened several times-- Trying simply to add 1 ppt presentation into another, both different templates. Should be simple, right?

I choose INSERT SLIDES/REUSE SLIDES, choose the file, right click the thumbnail from the 2nd deck, and choose INSERT ALL SLIDES and then... nothing.

Same thing when I copy/paste in slide sorter, they won't paste in, WTF?