If you've added subscribers to the sequence from a visual automation, then they will leave the sequence as soon as they get the last email and go to the next step in the automation. So if you then add another email to the sequence, they won't get it as they have left the sequence already.
ConvertKit have recently added an option where you can keep people in the sequence after being added by a visual automation. If you head over to settings for the sequence, you should see the option.