Automated Series Not Sending to New Subscribers

Automated Series Not Sending to New Subscribers

I have a problem with setting up automated series of emails. I set up a Smart Series with 5 different emails to be sent 2-3 days apart to new users of the mobile app I'm marketing. I have a workflow set up to land new users of the app into a list and then those new users receive the sequence of five emails. It worked fine for the first 23 users; they all got the email. But now the list has over 1300 (workflow works fine) but none of the new ones are getting the emails. I thought that was the whole point of the Smart Series. It still shows as only 13 - 23 sent. Any help with this?