Terribly ashamed to state that I didn't keep any record of the steps taken to fix the problem! I'll try to remember:
I switched from Feed by Mail to Automated add-on.
You set up at least one channel, and each subscriber needs to be linked to the channel.
I was already sending emails to the subscribers who signed up since I set up Newsletter, but the ones I imported from Mailchimp were not getting the emails.
Then under subscribers -> maintenance -> lists (tab)
you see a <select> which includes the channel from Automated add-on. In my case that is the only option, but it needs to be selected if you have multiple lists.
then click on "Activate for everyone". The Lists tab has lots of options for bulk operations on your subscribers, so it's very powerful (and dangerous).
Stefan was helpful, not sure why his replies are not on this thread, but I recall a few of his inputs, including that each subscriber shows a list of the newsletters they received, (Newsletters tab), and the list they should be on (Lists tab).