Rikk, thanks for your reply and suggestions. I had already deleted and reinstalled both programs, which didn't fix the issue, but maybe I should try it again. Before I do that, I will say that after I sent the reply yesterday, I looked further in this forum and found a suggestion to clear out the preferences file from the app data roaming folder, which I did. That did not seem to fix the issue, so tried the next recommendation which was to start LR while holding down the shift and alt keys. That gives you a choice to reset preferences. After resetting, I looked for another folder to try, and when I processed a photo in PS, it actually came back to LR! So I thought that must be the fix.
But in testing a few other folders, what I actually see is that I cannot get the photo back from PS if it starts in a Collection Set. I am able to successfully send photos to PS and back if I use photos from Collections, but not Collection Sets. I can also successfully send photos from imported Library folders, that aren't even in a Collection.
Unfortunately, I can't tell you which step in my process may have fixed the function from the Library folders and from Collections, but maybe others can try the shift-alt way of restarting Lightroom and see if that helps. Just close LR, and restart it while holding those two keys. You then get the choice to reset it to factory preferences.
I also have a feeling that I was able to successfully send photos from Collection Sets before, but I can't prove it. Now that never works, and if that's a new issue, it occurred after my last update. If you never could do that, then I learned something useful at least.
Here's the Collection Set "UK 2012" and the subfolders which are the Collections, to be clear about what I'm talking about.
Thanks.