The email associated with the account is not receiving any notification of a new sign up.

Please check why this email address, sales@rosberg.com does not receive notification everytime somebody signs up on the form. This is a paid subscription and we haven't received anything since June, 2016! Kindly check.

I checked the sent email logs for the mentioned email address and I see at this time that emails were being sent successfully.

But I did see that it was failing previously. It look like it was fixed already as succeeding emails were now sent.

You can review the sent email logs on your Form Builder as well.

User guide: How-to-View-All-Your-Form-Email-History

If the email is not coming, it is possible also it was in bounce list. You can check and remove bounce listed email too.

User guide: How-to-Remove-Your-Email-Address-from-Bounce-List 

Hope this help. Let us know if issue persist.