The vast majority of people who take action on a campaign through your call to action email will do so within three days of receiving that email. In order to increase your participation rate, send a reminder on the fourth day. You may even want to send additional reminders weekly after that.
Go to Messaging > Manage Broadcasts. You will see a list of your unsent drafts at the top, but below that are lists of previously sent broadcasts.
Look for the original call to action broadcast you sent under the appropriate section, probably Advocacy Calls to Action. You will see two icons to the right of each sent call to action broadcast. The icon furthest to the right will create a clone of the original, but you will want to click the icon with two arrows (Track and Resend).
You will then be able to choose to send a reminder to only the contacts who have not yet taken the action you originally requested.
Continue forward to make any necessary edits to the email on the Compose page, just as you would for a normal email broadcast.