the NF form sending BCCs is really a Ninja Forms issue, because it is NF that is not sending the BCC, not GD.
That said, we have a section in our docs about diagnosing email issues. The first step is to install the email log plugin to see which emails are created and what they include. Try that and check to see what is happening with those emails.
To open a new support ticket, first select the relevant forum and then click the "New Post" button. Example: You need support for the core plugin? Click on Core Plugin support and click on the "New Post" Button.