Thanks for posting! Can you check the email report for the email in question to see if any of these contacts bounced back? This would give us lots of information depending on the bounce type. If they didn't bounce and couldn't find the email in their junk/spam folders we may need to do more digging.
If this is the case please email us the addresses in question so we can look into this. Include a reference to this post in your email please.
We are happy to look into your email address on the account. For your privacy can you please email that address and any others you want us to look at to social_support(at)constantcontact(dot)com please? We can then take a look at the account to see if we can determine the cause. If you would like to speak to someone immediately about this, please feel free to call in to our support center and speak with our Delivery team.