Please note: this service is no longer available as of October the 2nd, 2017.
The way our "Twitter Welcome" service works, is that it will go through everyone in your "Follower" list and send them a message until everyone in that list has received a welcome message. After that it will only send welcome responses to new followers. It picks the followers out by random and has been designed this way due as Twitter doesn't currently provide us with information of when a Twitter account started to follow the account linked to the automation.
Once it's done this, the service will start send messages to new followers only.