Lightroom: Why is "Copy Settings" and "Paste Settings" not working in both Grid & Develop modules?

You could try resetting LR's preferences: https://www.lightroomqueen.com/articles-page/how-do-i-reset-lightrooms-preferences/. That magic spell often fixes things when LR's user interface acts up.

This question is from a long time ago, but I'm having the same problem, and figured out how to work around it.  I did reset my preferences, and that did not help at all.

Lightroom behaviour:
In Library module, right-click on any image in filmstrip, copy develop settings.  Right-click on any other image in filmstrip, paste develop settings.  THIS WORKS.

In Develop module, following the process as above DOES NOT WORK.  However, this does:
Right-click on any image in filmstrip, copy develop settings.  Click the target image (in filmstrip) to make it the active image.  Click anywhere else in Lightroom.  Then, right-click the active image thumbnail in filmstrip, paste develop settings.  THIS WORKS.

Why it works this way, and why the extra clicking is necessary, remains a mystery.  But through trial and error, it does seem to consistently work with the extra clicks.  Hope this helps.

The difference in behavior depends on whether you're in View > Loupe mode, not in Library or Develop. When in Loupe, LR is very inconsistent about which commands apply to all selected photos and which apply just to the most-selected photo:

https://feedback.photoshop.com/photoshop_family/topics/long-time-pet-peeves?topic-reply-list[setting...

(It's a mess.)

@Shantastic,  I was able to replicate the same issue on my PC (LR9.2 on Win10)

As well as the 'extra' click method you described, I also found that the "paste settings" step shows in the history panel and if you click back to the previous step then click the new "paste settings" step the settings are applied.
Or if you use the Copy & Paste buttons in Develop vs the right click menu it also works ok.

It works ok from the right click menus using LR9.0 on my laptop. All tests were from one master image to a single recipient image so the messy LR copy/paste behaviours should not come into play?

There is also a similar issue with the previous button not always applying changes even though a history step shows. However, that behaviour is random vs this one  - there is already a bug report open for it.