At this time, you do not have the ability to add email touches to a campaign for recipients already in flight. The email touch would have to have been added prior to when you added the recipients and sent out the campaign.

However, this means that any new recipients added to the campaign, moving forward, will receive the new email touch.

For any recipients that won't receive the follow-up email touch, as a workaround, we suggest creating a single email touch campaign with the follow-up email touch that you wanted to send out and adding those recipients to the campaign.