Cannot copy/paste some slides from one PowerPoint 2016 to another (some work and some don't work)

I needed to copy 20 or so slides from one .pptx to another .pptx. Both presentations were built in PowerPoint 2010, and we've upgraded to 2016. Both presentations have been used over and over with no issue since the upgrade. 

With both presentations open, I selected the slides to copy from Presentation 1, and I dragged them from one monitor to the other, like I do frequently. Nothing happened. Nothing happened. PowerPoint blinked, but nothing pasted. I tried dragging individual slides over one at a time, but only some of them worked. Since one presentation was on SharePoint, I made a local copy, but that also did not work.

None of these different ways to paste worked:

1. CTRL+V
2. the paste button on the ribbon.
3. paste using the destination theme
4. paste keeping the theme
5. Paste As

None of these different ways to copy or move worked:

1. Using Copy from the ribbon
2. CTRL+C
3. right-click copy
4. Using Cut from right-click menu and from CTRL+X

The Solution:

I found that slides with review comments are the culprit. If a slide contains a review comment, it allows you to copy it, and paste options are available, but nothing happens. If only one slide in a group of 10 slides I want to copy has a comment, the entire copy job fails. 

To "fix" and allow me to copy these slides, I removed all of the comments from the slides that had them. Once every comment was removed, the copy/paste job worked properly. I could copy individual slides as well as the entire group.

I hope this helps anyone who has phantom "my slides won't copy" issues!