Not receiving email notifications from any of my groups

It's possible (I have my FB notifications just about 100% off so I'm not certain) that the group new post notifications have a differing sender or a different email header, possibly both. Your email program might be binning the group notifications in part, in particular if they all have the same (or nearly the same) email header verbiage.

Have you checked your spam folder and/or contacted your email provider to make sure that you get the emails you want to see? Tell them that there are false positives as they should be able to tweak the settings.

I wondered that too, but I see no spam in that folder, and I never set my spam settings to "on." Even before it hits Outlook, I see no spam in any folders by using Cox web mail. I wonder if I need to bring up to Cox, my ISP?

I'd try 'em (nothing ventured, nothing gained). FB might be smacking against some unseen limit that they've got, or something in their algorithms.