S2 would send notices to registered users (just 2 of my personal addresses), but nothing to Subscribers. I tried everything.
Finally went to my hosting service (ICDSoft.com) which has an excellent support service (SureSupport.com). They worked on it for a few hours last night, at no cost to me. Finally they came up with this:
We have sent several test messages and discovered that when the "Send Emails for Private Posts" delivery option is enabled, the plugin does not include the Public Subscribers in the BCC list. We have set the "Send Emails for Private Posts" option to "No" and the test message has been sent to the subscribers.
Please let us know if we can be of any additional assistance to you.
Why that happens I have no idea. But my problem is apparently solved. Awesome!