Please note that on July 2nd, 2013, Twitter changed their terms of service and outlawed automated following back of people who followed you first.
You can follow an ongoing discussion about the matter here:
Unfortunately we have no choice but to modify our system to comply with Twitter’s new rules.
Hence, from Monday, July 8th, 2013, the manual vetting option of new followers will be automatically enabled on all accounts that currently have the auto-follow option enabled. If you do not manually approve the follow-back, as Twitter now requires, then the follow-back will not take place.
If you are not using auto-follow-back but are just using the auto-welcome-DM, then your account will not be affected. Auto-welcome-DMs will continue as before without any change, and you do not have to do anything.
If you are using both auto-follow-back and auto-welcome-DM on a Twitter account, then you will need to manually approve each new follower, as per Twitter’s new requirements, for that new follower to be followed-back and to receive the welcome-DM.
If you have any issues with or concerns over this new Twitter policy, please contact Twitter directly via support.twitter.com or by commenting on the discussion thread linked to above, or both.
Should Twitter in the future decide to again allow auto-follow-back, we would be more than happy to restore the auto-follow-back service that you have found so useful for such a long time. We’re as dumb-founded by Twitter’s decision as you are.