When going through your email reporting data, if you feel like you're missing sales attribution to one or more of your emails, one of the first things to check is that EVERY link the email that gets sent, has tracking parameters (UTM values).
We recommend opening the email and looking at each link. Even better, would be to send yourself the email, so you can click on the links. When you click the link, you will want to verify that the page you end up on, still has UTM tracking parameters.
Note: We've seen many times where a link in an email ends up being a redirect link and when the link redirects, it strips the tracking parameters from the URL.