If you've added a new autoresponder email in your autoresponder series, the email is not sent to subscribers who were already on the list before you added the autoresponder message.
The easiest way to send the email to your existing subscribers is to create a segment of your existing subscribers and then send them a campaign with the content of the AR email.
Let's say you added a new autoresponder email on 5 August, 2012.
Here's how you can send that email to your existing subscribers:
1. Go to the Lists tab and click on the list name
2. Click on Segments
3. Click on Create New Segment
4. Give a name to your segment
5. Under Segment Rules, Click on add new rule --> Subscription Date
6. Select "IS BEFORE 08/05/2012"
7. Click on the Create Segment button.
Now you've created a segment of subscribers who were on your list before you added the new autoresponder email.
Now, go to the Campaigns tab and click on Create A New Campaign.
Select the segment you just created, and copy paste the content of your autoresponder email to send the campaign.
Finally, go to the Lists tab, and click on your list name. Then, click on Segments and delete the segment after the campaign has been sent.
Repeat the same step for every autoresponder email you want to add.