I had this happen to me recently too. When I exited out of the mask panel and then returned, the add and subtract buttons came back. Have you tried this? If that doesn't fix it, what happens if you quit Lightroom Classic and then reopen it?
Hi Theresa – thanks for your reply. Yes I have tried both of those things, but with no success. I am considering uninstall/reinstall as my next move.
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Just an update. I did the uninstall and reinstall and the masking is now working as it was prior to this issue arising. Very strange, but at least it is back!
Your second screenshot shows three collapsed masks. The buttons only show when the mask is expanded, so you see the components that make up the mask. Clicking on it once more, or double clicking may be all you need to do next time.
The double clicking worked for me.