@WolverineX61 do you have a Salesforce contact or lead being sent with the call to DocGen? In order for Salesforce templates to merge data into an email template, contact or lead is required. 

Oh "DocGenVinnie" (classic!)

We've been running merge fields in our SFDC Email Templates for years without adding a contact to our emails.  When did this change, for merge fields you "have" to have a Lead/Contact associated? 

We don't use the Contact with MIC because we don't want users to look up Accounts/Contacts using OOTB SFDC funcationlity; the look up is not not robust/comprehensive enough FYI.

Let me know, you know how to find me :)  
- tjb

Trying to resolve for this same issue - when you say "sent with the call to DocGen," where exactly do we need this added - in the Delivery Options, or on the Email Template, or somewhere else?

We are using a custom field as the main field, and this email is for internal users - not sure why Contact or Lead info would be needed. 

This is something enforced by Salesforce.  We have customers work around this by setting up a contact that get's sent internally.  You would have to add the internal as an addition to and uncheck the internal email box.  

Not sure how complex your email template is but another option is to use Word to Email for an email template.

https://help.nintex.com/en-US/docgensf/Templates/EmailTemplates.htm

Thanks Vinnie -- 

We ended up creating a Word template.

I believe the below article was added or edited to explain the issue as well. I would suggest possibly adding info in theWord template how-to article you posted that if you're having issues with merge fields not populating to pursue that option. 

Thanks to you and your team for the quick follow-up.

https://community.nintex.com/t5/Best-Practices/Drawloop-Email-Merge-fields-not-populating-on-a-Salesforce-Email/ta-p/111980