Thank you for reaching out to the Community.
When you say you've been using the same email template have you been making a copy of your previously sent email each time? Or do you have a master template you copy from?
I recommend reaching out to us at social_support(at)constantcontact(dot)com with the name of the template you are having the issues with and the name of the email client (Outlook, Gmail, Yahoo, etc.) you are seeing them. We can investigate further and see what might be going on.